Follower not binding?

We need more information:
When was it last taken apart and deep cleaned? Are all the screws tight? What ammunition is he using? Is it always the same ammo? When he shoots it at the range are the cartridges fed one at a time by hand or does he put them in the magazine and lever them into battery when ready to fire? Does that model have a cross bolt safety? On the fourth round,does he pull the trigger and nothing happens or does he pull the trigger and hears a snap but the cartridge does not fire? Is the primer dented on the fourth round?

Thanks for the questions, will discuss with Bro.