I have two 13rd mags right now for my Glock but they are NFML and the +2 extenstion I bought says it will only fit FMLM. Is this for a reason? I am trying it on my mags but it doesnt seem to fit right unless I only use 13rds. When I get to 15 it seems like it will fall off the bottom with enough force or if slapped up into the gun. Anyone esle have this issue??