I have not run across this particular question before and do not know our relative experience with Taijiquan. As a result, I cannot give you any certain advice. What JP says in his post sounds accurate, elegant, and pertinent; however, I would be surprised if that would be the sole reason for your difficulty.
I also agree that the rotational aspects of the posture are important; however, I would caution that this can be overemphasized. As I understand it, Parting Wild Horse’s Mane and Flying Diagonal are not supposed to train quite the same energy. The former trains the lifting energy of Ward Off, whereas the latter trains the rotating energy of Split/Rend.
Although I cannot really diagnose your problem, I can give you my general take on such problems in a sort of schematic way. I say “schematic” because it is very hard to describe the important part of these things in words. It is more a matter of relationships than limb positions. If we were in the same room, I could exhaust what I would have to show you in about sixty seconds worth of discussion and demonstration. Just because I try to describe a principle does not mean I think you are ignorant of it. I just like to provide the complete context of my thoughts and not assume that everyone has the same take on things that I do.
The classics say something like the following: “Power is rooted in the feet, generated by the legs, controlled by the waist, and expressed by the hands and fingers.” Your question seems to focus on how power is generated, but let me comment on some of these other terms to clarify what my take is on them.
If power has no root, it cannot be brought to bear. It becomes irrelevant. Think of trying to push someone away from you while you are standing on roller skates. Having no traction means that no pushing power can be brought to bear, regardless of your strength.
If we assume your root is okay, we can then examine the issue of the legs. My understanding is that power is basically generated by the legs, but “issued by the spine.” My take on this is that the spine provides the inner framework for the “trunk” of the body. By “trunk” of the body, I mean the “chunk” of your body that contains most of its mass and that tends to move as a solid unit. Basically, this is everything but the legs, arms, neck, and head. The reason why weight shifts are so important is that they involve the large amount of power that is necessary to move the mass of the trunk. This power should be continuously harnessed.
The same power that moves the trunk of your body is the power that is most relevant for Taijiquan. The legs can move the trunk backward and forward between them within a “stance” or can rotate the trunk with or without a linear weight shift. If you think of the principles involved in ordinary physics and the amount of force necessary to move the trunk of a 200-pound person backward and forward within a stance or to rotate the trunk, you will know what I am talking about. This same force can, of course, be “focused” through an arm, fist, or palm.
If the movement of your trunk is okay, the next issue is your arms and hands. In my view, Taijiquan does indeed use the muscles in the arms and hands to generate power; however, I believe that thinking in these terms is mostly unnecessary and usually counterproductive. The main issue is to avoid using your arms in a way that would prevent the force that is being applied to your trunk from also reaching your arms and hands. This does not mean leaving your arms limp. The arms can add a little power in an integrated way, and the hands will then mostly “manifest” the power being generated by your legs.
In my description so far, I have left out the “waist.” In my view, waist “theory” may be incredible complex, but waist “practice” is not necessarily so. Here, I will try to avoid theory and give you my take on practice.
There are at least three issues with aligning the waist correctly. First, you can think of the waist as a joint. If it is unduly bent or floppy, it will make your legs and spine behave like two sections of a broken stick, with the waist serving as the broken point. Power applied to one end of the stick cannot reach the other effectively. If you twirl one end of the stick, the other end will twirl as well, but it will lack any power or integrity because of the broken section. Make sure that the positioning of your pelvis and lower back do not act in this way.
Second, you can think of the waist as the foundation of the spine. If power is to flow up from the legs through the “waist” to the upper back and to the arms, you do not want a “bend” in the structure. Imagine the difference in the transfer of power when you push something with a straight stick and when you do so with a bent stick. The “straight” spine or stick provides a stronger structure. A common defect most people must fight is to maintain too much of a sway in the back that blocks the transfer of power. The muscles in the back are not designed to support each other in this position.
Although the metaphor of a bow can be applied to the spine, my belief is that this metaphor does not refer to the utility of a pronounced bend in the spine. Your pelvis will naturally flex back and forth somewhat in accordance with your stepping and should not be held stiffly; nevertheless, you want to make sure that it is not out of position to transfer force from your legs up your spine. Some people talk of making sure to have the tailbone hang downward. Others talk about tucking it under slightly.
Third, you can think of the waist as the “core” of the trunk. In my view, the trunk of the body is too large an area to focus on with your mind. It is hard to visualize how to “target” it and “steer” it precisely. Both the upper torso and the pelvis have some range of movement that is independent of the rest of the trunk and so do not provide good guidance for what is happening to the trunk as a whole. The best solution, in my view, is to focus on an area between these two, i.e., on the “waist.” The waist is the handle by which you orient the body, just as the middle of a bow is the place where you hold the power of a bow. To use another metaphor, one could say that the waist is the steering wheel of the body and the legs are the motor. Power comes from the “motor” in the legs, but must be “steered” by the waist. Otherwise, it is like engaging the clutch of a car and wondering why there is a decrease in power.
You also mentioned “hips” in your post. While it can be useful to talk about hips during demonstrations or during live teaching, it can be very confusing in print. Depending on context, “hips” can refer variously to the joints that link the thigh bone and the pelvis, the inner section of this joint, the bony mass at the side of our bodies (above which our belts lie), or even the pelvis as a whole as seen from outside the body. If you talk about how to move the hips, things can get even more confusing, since movement through space can be caused by a whole number of muscles in the legs and pelvis region. For example, if you bend and straighten your knees in alternative fashion, you can wiggle your waist without really using much of the muscles involved with the hip joint.
I would summarize by repeating the saying that if you have an issue with power, “look to the legs and waist.” If your legs are strong enough to propel and rotate your trunk with speed and power, there is no reason that this power cannot flow through to your arm.
I hope this helps.