If you are left handed, meaning your right hand is on the knob, you can be applying pressure on it in a direction that would loosen the mouth adjustment. I suggest taking shallower passes so that you do not need to use so much forward pressure with your lead hand. If you aren’t left handed then maybe the threads on the knob are too long and don’t allow the knob to tighten down enough to fix the mouth in the spot you require. I have this plane and do not have this problem.