Like the subject says, I have a P1 which will not feed the last round in one magazine (unmarked except for a made in Mexico stamp). It doesn't jam or stovepipe. It just sits there at the top of the mag. I have another mag which works fine. Any ideas of what to look for? Or should I just give up...

Took PP to range and it doesn't like Remington FMJ or PMC. Range smith says older German guns like european ammo like Fiocchi or S&B. I see lots of folks here feeding their Walthers non-european ammo. Any suggestions on steps to take. When slide is racked, the ammo does not move up.