Check the right hand extractor on the bolt. It can sometimes pop out of it's seat in the bolt slightly. It'll still be in the slot in the bolt but will be sitting slightly forward and outward from where it's supposed to be.
What that does is is then limits very slightly the amt of rearward travel distance the bolt can be pushed by the slide.
Since the bolt doesn't quite move it's full distance to the rear,,the carrier which is cam timed to function along with it,,does not quite lower itself all the way to allow a round to load from the magazine.
If it is the issue and the extractor isn't damaged and it's coil spring behind it is still in tact, the extractor can usually be pushed to the rear and it will snap back into it's seat and lock into position.
Also check the cartridge cut-off's on the inside forward walls of the action. Either side of the carrier. They manipulate, halt and then release the round in the magazine at the correct time for loading onto the carrier.
If one is bent or the somehow jamming the carrier that could be the problem.
From memory, I think the one on the right hand side (looking at the gun turned upside down and looking down at the carrier and loading port is the one that is actuated by the carrier itself but I might have that switched around)
