Just re-read the OP's "symptoms", it sounds like it feeds o.k., but then doesn't have the force to eject very far ( empty case hung up on the bolt) and not enough force to strip another round. I'd take apart the gas block and inspect the gas bushing and tube for obstruction/restrictions while you have it apart to clean. Normally the Mini's are so over gassed that they are super reliable, for one not to be, given good magazines and good ammo, seems like it might not be getting enough gas to cycle the action. Much less likely but something that might cause it to be slow in stripping another round is a weak recoil spring, but I've never heard of a spring being that weak. And also a possible cause of malfunctions could be using buffers that are too thick, but again, not as likely as the carbine not getting enough gas.
