Do these seats have cables, like the 960 power seats?
If they do, one problem was shrinkage of the mechanically-driven cables, which reside between the seat motor and the gear which drives the fore/aft rack on each side of the seat. When the cables on 960-type power seats shrink, it's common to find one side (let's say LH) of the seat will move fore and aft, while the other (let's say RH side) won't move at all. This happens because the square mechanical drive cable end has shrunk back from the square driven end at either the motor end (most common) or the gear end.
Robert DIY on YouTube has found a simple and brilliant way or repairing these shrunken cables. His solution worked for me on a 96 960. That said, I don't know if your XC70 seats have a similar mechanism; after all, Mother Volvo did leave some car components unchanged for decades.