How'd you get glocks in 1978 when they were introduced in 1982. And that's if you were working with the austrian army.
As for OP's problem, I'd look at cartrtidge overall length vs. mag length/depth. My guess is the tip of the cartridge is dragging, and this is not a problem with FMJ, but is with the polymer filled tip.
Normally I'd be with you that it's a mag problem, but given the current state of the ammo industry, QC issues are much more likely, so I wouldn't rule out ammo issues as well.