could be the ammo
the last case i bought is crap, shots are up n down from one round to the next, and about 1/3 through the crate its had 5 misfires, fully dented primers but no bang, even a couple with extra thick paint that mess around with the cycling
in the case i shot off before it was completely 100 percent for firing and cycling
both these cases were bought at the same place
another thing is that there could be some dirt in it, my cz was pulled out of a sealed box and had alot of grit (read sand) in the grease that covered the reciever