you can overcome this problem by grinding the little round locating lug off the end off the bearing shaft. this is where the get stuck. usually when the car has been running for a little while and the shaft expands. another thing you can do when it gets stuck is just loosen off the 4 screws that hold the pull start on and give it a gentle tap. that will do the trick.

this will happen quite regularly as it is a poor design. infact the mach .28 is pretty much a money waster imo.

hope this helps