Colin
If its not wrecked, grab it before their Prozac wears off

IIRC ( I have a S7, no x-feed) the half nuts are on dovetails like a slide, so it may be the adjuster screws are too tight, or there is gunge in 'em.
Wiil try to find a drawing if you like .. ie in the manual .. or ....I think I've seen it on the web somewhere .
No big problem probably ..
EDIT Owd on .....Scanning as we spoke ...
Part #20 is the gib-strip wotsit, and 29 are the two screws that hold 'em on.
Not sure whether the power x-feed are the same tho. ??
PS. I also seem to remember on my ML7T, you can make the operation very stiff, by over tightening the (nyloc ?) nut ... #26

Dave BC