As John says, it's worth having a feel underneath the seat, in particular checking the runners for foreign bodies.
I did have a similar problem on another car where the claw which engages with the rack came adrift from the shaft on one side. The spring in the mechanism held it firmly in the rack and the seat wouldn't move. After removing the seat, a couple of small welds to re-affix the claw to the shaft solved the problem.
Could well be something similar on yours, it's worth having the seat out to see what's going on.