Originally Posted by
liampboyle
Ok, Shimano deore XT shifters, I can't see any frays in the cable, and if I pull on the exposed part of the cable it does pull in the rear derailleur.
Undo the cable at the RD, does the RD move? if no, it's seized and need replacing (unlikely)
If not, expose the inner at the shifter end, and hold it while using the shifter, does anything happen, if not, replace the shifter (again unlikely)
Is the cable hung up on anything in its routing?
Most likely from the limited info given, the cables need a lube, or even better, replacement, you can buy a Shimano MTB deraileur cable pack which include all inners, outers and ferrules etc (The cable outers will be marked SP41)