There is a spring under the seat
between the rails. It connects the seat tray to the floor to provide some tension when you slide forward and a little help when you slide to the rear.
See if that spring is still attached and check for a bent lip at the end of the rail. I seem to recall the factory had a "retainer" built into the rail to keep it on there during noraml use. I could be wrong about that, it's been a while.
Also check for crud,loose change, etc in the track. The tracks are spot welded to the floor on the early cars, so you have to slide the seat all the way forward to get it off. I pushed the seat as far forward as a could with the seat back pointed towards the steering wheel. Then when it would go no further I reclined the back a shoved some more. Eventually I got it off.