Couple of things to try:- Firstly (and apologies if this is teaching you to suck eggs!):- When you kick a bag or pad, don't kick it, kick through it. Try to visualise that you are actually kicking the other side of the person holding the bag/pads, let your leg swing through. Secondally, twist the other foot. If you are doing a back leg mawashi geri, the dynamic comes from turning on the ball of your front foot, getting your hips around and finally your leg snapping out. It is this that allows you the coiled spring effect to hit the other side of the bag. Lastly you could try some bungees. We use than for conditioning and speed. If you put one around your ankle and fix the other to the floor, or have someone hold it you can kick against the stretch of the band. upi can then also add a pad to hit.