Having recently removed my seats and butchered up a set of frames to fit buckets, I had hoped to be able to answer this. But not sure I understand what’s happened, photos definitely needed!
The seat is kept in place by a pin which slots through a hole in a central bar. When you lift the lever at the front of the seat, the pin is pulled out of the bar which allows forward and backward movement. When you release the lever, the pin finds the nearest hole, slots back in, and the seat is locked.
Tension is kept on the lever by a very small spring at the rear of the seat, you can just about make it out in this photo. If this has snapped or come loose, the pin would not be forced back in once you’d released the lever, meaning the seat would continue to slide. I’m not sure if just pushing down on the lever would return the pin to the lock position in this scenario.
Just as a side note you can’t see the pin referred to above in the photo below as the pin section is found attached to the car beneath the seat. What you can see is the spring and central bar with holes in it. Very hard to explain in writing I’m afraid.
Alternatively, it could just be that your rails need greased. Mine were extremely stiff when I took the seats out which made the job a little tricky, took some serious effort to slide them back far enough to get them out.
