I guess it's possible that one mag could have a weaker spring than the other...weird but possible.
1. Did you try both mags when you were at the range that day?
2. Did they both give you the same problem?
Making sure the gun and mag is unloaded...insert a mag and rack the slide a few times manually. Do the same with the other mag. Does the slide lock back each time?
I reject your reality and substitute my own.