As PanteraPatt mentioned, a 6 ft long 2x4 against the rear firewall will move even the most stubborn seat, once the locking lever is released. I once had to do this for a converted Pantera that had Ferrari seats which used flat sliders that drag on the carpeting, in place of Pantera rollers.
The seats were bolted down directly on the carpet without the stock Pantera front & rear aluminum rail-spacers, probably to slightly lower the seats. Doing this causes any adjuster rails to bend as the carpet compresses during bolt tightening, and fouls up all seat movement.