Well, since they work fine with real ammo, it's not a really big deal. Frustrating, but not necessarily a real problem. However, I'd make an effort to do a real thorough cleaning job on the extractor area on both pistols in case accumulated crud may be contributing.

I'd also suggest actually measuring the dimensions of the 'case head' area of the snap caps and compare them to actual case dimensions. I'd also suggest making sure the snap caps can fall out of the chamber by their own weight if the barrel is turned muzzle up. That it only happens with the snap caps suggests they may be the issue even if they do work in the brand "G".