I suspect some sort of debris interfering with the action. My first guess would have been something like a dead insect in the spring or barrel compartment, but there could also be a crumb of something under the "foot" plate or whatever the spring base is called.
Those boards are beasts to dismantle, especially the big ones, but you could give it a thorough cleaning and check everything out so that you don't start losing more action.
If it is an F-122, cut off the sliding tabs and do a bolt mod. It only takes half a dozen bolts, and you can get them at a hardware store, unlike the 50 tiny special-order bolts you need for an M.
I did a wiki on that, but I have not checked to see whether it is still around on this side of the "event horizon".
edit: oops! I didn't see that you were talking about XTs and ATs.
How do we find wikis on site site, or are they gone?