I had prepared a nice long post about possibly the left ejector dragging on the single and also slowing down the right on a pair.
I guess that's not the case!
Ejector spring(s) could be non matching replacement.
Is the small roller in the ejector forend assembly in place and free?
It's cross pinned between the two ejector hammers. At the very rear of them and down between them in the forend iron. You can see it barely if you look down at the rear taper of the hammers.
That roller secures the hammers in place and allows them to run smoothly back and forth. Missing or makeshift part there will cause slowdown or stop in the action.
Can't think of anything else off hand other than the left ejector kicking too soon and the shell nipping the top of the frame on the way out slowing it down. Ejecting the pair might have some effect on the right side if it's springing back in that moment.
All just a guess of course..
