Well I've had this problem once.
Did you happen to take out the white "air restricter piece"?
That piece kinda acts like a plunger stop, so when you fire your gun
the plunger shoots forward more then its supposed to and gets stuck.
Yes the PAS (pump action shotty) if very good indeed, I added a LS spring to mine with a barrel replacement and it hits 100". I got mine at Academy for $30.00. Its a little pricey but totally worth it.
If you go to Academy I got mine in the toy section.