Take the seat out , you will find a bolt on each corner holding it down most likly size 9/16 or 14 mm un plug wiring or anything else that might keep the seat from coming out and turn it upside down and examine it for a broke spring or latch on the track it slides on when you pull the handle to adjust it he seat ,slide the seat back and forth on the track appling pressure as you set it in different positions ,you will find some reason there why it is not locking in to the track .
You might have to replace the track if it cant be repaired , you can find one at a used auto parts , it would be expensive if you bought it new from the dealer.


