May have found the problem and it
may not be me!
I was comparing (yet another reason to have more than one!) to a newer 870 and I noticed that the carrier seemed to drop easier on this '55. It is possible that the weight of the shell combined with the rapid shucking was bumping the carrier down before the shell was started into the chamber.
I could cycle the gun slowly, stop just before the shell had entered the chamber and with a gentle nudge, push the carrier down enough to mis-align the shell so that it would hit the bottom edge of the chamber and not feed, essentially recreating my trouble.
I figured maybe a weak carrier dog follower spring but after I took it apart it looked OK, not that I have any way to measure such a small spring.
Anyway, I took a good look at the carrier dog and thought that maybe it looked a little too smooth on the step that rides against the carrier dog follower, so.....
I took another carrier, complete with the carrier dog and swapped them.
Now, as I cycle slowly, I cannot push the shell down and the carrier does not drop away until the shell has started into the chamber.
I will test this week to verify but I am hopeful that this was a problem that can be attributed to the mechanics of the gun and not the mechanics of the shooter!
Stay tuned....
Mike