What kind of magazine were you using?
sounds like the tab on the back of the mag isn't seating all the way to the selector stop when inserted.
Could be that one mag is slightly longer from back to front.
Filing a tad off of the inside back of the magwell could solve this issue or you could just stick with mags that work in that rifle.
21 Club #150 California Glockers #150 Kalashnikov Klub #150 Black Rifle Club #377