one quick check is to loosen the muffler just a bit and see if it revs then
an easy mistake could be putting base gasket on wrong, which can block transfer ports
most common mistake is to order a top end set that has diff barrel height, diff port height, or diff piston - if you find no other reason, then take it off & carefully measure all between old set & new
that, and debris falling into bottom end are some reasons I rarely rebuild top ends when new motors are so cheap