Watch how the leg and hips move first, then the shoulder and finally the bat. That is how a power punch is thrown – the hand or fist is essentially the end of a whip that begins with the feet.
You can perform this exercise as a partner exercise (what I use since I don’t have a concrete or cinderblock wall). It’s not as fast but you can vary it with distance between partners (the further away the harder you have to throw but the longer the time) as well as weight of the ball.
It may not look like a lot of hip torque but you’ve got to try it to feel it.
Looks like the hammer was to heavy but I hope you get the idea.
The following video is another is another illustration (the video is rotated 90 degrees but again, I hope you get the general idea),
These are potential supplemental drills to hitting the bag. In the end bag work is best (if your time is limited spend it on actual technique training) but these other training methods can provide you with the “feel” – how it feels to develop power through the hips and leg for rotation so that you know how it should feel when you punch the bag.