Rimfire Cartridges Just Aren't That Well Sealed
Over the years, the only problems I have had with factory ammo failing to fire have been .22 LR. One batch I had kept in the trunk of my car--and appears that they got wet. Yesterday, I took some friends out to familiarize them with handguns--and the only failures to fire were .22 LR in magazines that I sometimes carry in my pocket. About half the cartridges would not fire, even with repeated strikes. (The American Arms PX-22, being a PPK clone, has a double action trigger.)
I'm guessing that sweat from carrying them in my pocket had neutralized them. While this was a good drill for teaching my friends how to clear failures, it was a sobering reminder that any .22 LR pistol that you carry as a backup gun should be fired every month--so that you always have fresh ammunition in your gun.
There's a school of thought that says .25 ACP makes no sense, because it has about the same power as .22 LR, but costs 10x as much. The difference is that .25 ACP, being a centerfire cartridge, is probably much better sealed.