I had this problem earlier this year and found that the plastic trim piece around the seatbelt had broken, allowing the seat belt to retract too far into the seat. There's a little button that keeps it from doing so on the belt. The only way I could get the seat to go back up was to unscrew the seatbelt at it's lower mounting point under the seat. That wasn't too hard to do, just needed a socket and some flexibility along with a good sense of touch since you really can't see under there with the seatback in the down position.

HTH.

Paul