So are the super socks failing to feed onto the lifter from the magazine, or failing to feed from the lifter into the chamber?
How long is the loaded super sock shell?
I have heard the 870's are not fond of short shells like the Aguila mini shells, and have to have the lifter modified. If they are not on the shorter in OAL, then look at the rim. Perhaps use a marker and cover the brass
My other suggestion is to get a set of calipers, and measure all of the external dimensions to your less lethal shells, and then compare the measurements to some of your other shells.
(Why not ditch the less lethal, and stick with buck and/or slugs as God intended it?)
Quando Omni Flunkus Moritati