Coaxial transparent air cannon QEV piston detail 100psi 1/8th speed 15fps

I can't really think of a reason why the piston returns to the original position!! any ideas?? I made a transparent QEV coaxial cannon out of the preform of a bottle. This was inspired by a Nighthawkinlight's project, although I added a transparent barrel and had to make everything work with a wooden piston and o-ring seal. The breech seal is just a piece of rubber from the inner tube of a bike tire. Thanks for watching!