Redneckguy
The Remington 870 is about the most proven pump shotgun around. I have trained hundreds of officers in its use, with few problems. "Short shucking" the action can lead to a jam, but not the issue you seem to be having. If the Kent load is the only load you are having problems with, then that points to that load. Maybe the shell rim is undersized? If the shell is sticking in the chamber (but other makes are not), that also points to the particular shell. It is possible the extractor springs are weak, but I doubt that is the problem on a gun with 100 rounds thru it. Might want to switch to a different brand shell and see what happens, chances are it will be reliable as it is with your other loads.
Larry