[Dynamo]

make sure that the piston head is seated properly, and that the barings/bushing on both the inside of the piston and the back on the spring guide are seated properly.
if not, the spring may reach full compression befor the piston comes off the sector gear.
that is the only thing i can think of that would cause your piston from fully moving back.
if you have installed an extra bushing behind the piston head, it would cause this to happen.