Its possible the locking mechanism adjusting spring/“cable” has come apart. Have a look under the seat for a wire hanging down or sticking out.
There is a small wire coil spring to hold the end of the adjuster transfer wire (to unlock both rails).
Sometimes the small coil spring just backs its self off but is still loose on the transfer cable. (from the G”s of a Dodge).
Pinch the two ends back together and slide the small lock spring back in place.
or…. Locking mechanism simply needs a drop of oil.
The seat adjustment is similar for bench or buckets just a longer cable for bench.
