Picked up a ppk/s "factory demo". It worked fine for about 400 rounds, and then it woudl not stay open after last shot. Used all kinds of different ammo, with same problem. If I had the mag in there, and pulled slide back by hand, would stay open. Also, if I put in a fully loaded mag, draw back the slide to chamber a round, it won't always take the round with it. Same problem with different ammo. However, If I take and empty mag, pull back the slide, drop the mag and put in a fully loaded one, there is no problem. Any Ideas?