If running the stock feedneck, it could leave a gap that the hopper is filling, but i believe that the oring detents are the culprit. They aren't strong enough on their own to give enough resistance to stop the hopper.
Again, stock replacement is a 010 oring. I suggest getting any finger detent, like what Bob Long, Planet Eclipse or Empire uses for their guns, and just shove it in there. You should trim it to clearance the detent cover, but it should be fine otherwise.