Since it's used, check to see if the serial number on the barrel matches the serial number on the receiver and slide. It sounds like there is an issue with the ramp, and one reason for that could be someone swapped out the barrels for whatever reason and it wasn't fitted properly, and it gets out of sync now and again. or it needs to be polished.
I'm pretty sure there's also another cause it could be but I can't remember what it entails exactly. Someone here will most likely have an answer that explains the real reason. If all else fails call CZ-USA and ask their gunsmiths for an opinion.