It's primarly a weight transfer issue. A lot of punchers and kickers just can't effectively transmit their bodyweight. They punch with their limbs and not the body.
Turning the shoulders is a way to forcibly engage the body but from the ground up, weight transfer must be correct. That means, proper foot placement, distance, proper footwork when punching and proper cns firing sequence.
Some people are all over the shop with their cns. Strong as fuck in the upper body, strong as fuck in the lower body but come time to punch, they fire at the wrong points or freeze at some points in the sequence.
SOme are completely stagnant down low for example and they only work from the waist up, neglecting the hips, quads, hams, glutes, calves and toes into the whole equation.
A little technical but nonetheless important.
To remedy, quit trying to punch hard and work on proper technique slowly. Gradually build up speed and repeat as often as possible to forge the cns patterns for proper punching techique and firing of muscles. practicing over and over infront of a mirror or better yet lightly on a partner is effective.
Once technique is down pat, speed is at it's limit, add force. Power is force x speed.
Another thing to remember is to not just aim for a target but to aim through the target. Not a pushing punch through but rather a snapping powerful punch through the target, like the target is just paper in the way of the actual target 10-20 cm after or in the case of the Ike ibeabuchi's and David Tua's the real target being 1 meter through the actual target lol. I believe Kostya also practised this type of technique where his aim was to shatter everything beneath the target and not the actual superficial target.