As a general rule, if a gun you're unfamiliar with is failing to lock back the slide, the problem as Dinger pointed out is probably the shooter.
The best way to test this is to load all of your mags with one round each. Shooting the gun left-hand only (regardless of whether you're right- or left-handed), shoot each magazine. If the gun locks back every time, it's not the gun and it's not the magazine, it's your grip.
You do it L-handed because most guns have their slide stop levers on the right side. Shooting with the left hand makes it harder to interfere with the lever. For guns with ambi levers, it's not really as important, though it's often useful to shoot it with your weak (non-dominant, support) hand.