Bullets would not stay in magazine - ProMag Sig Sauer P938 9mm Luger Pistol Magazine

10 round mag - I tried only NovX Engagement Extreme Self-Defense Ammunition 9mm Luger 65 Grain ARX Lead-Free because that's what I use in my P938 for a great combination of low recoil and stopping power. However, these bullets just would not stay in this magazine. I don't know if it was the magazine's fault or the combination of magazine and bullets, but it didn't work for me. To be clear, I'd load bullets in and they would just pop out. I've loaded many thousands of rounds in many mags and never saw anything like it.