Well first try operating the lock mechansim with a screw driver with the door open. Give it a good WD40 ing and a few whacks with a rubber hammer. If that doesnt do it its probably a new lock required. BE WARNED though RHD cars have RHD door locks and LHD lock will probably upset things. I had a whole lot of poo caused by the same fault on our Seat. I wish I had jusr paid up for a new Seat UK spec RHD lock in the first place even though it was silly money. Even a decent quality £90 lock from GSF was a left hand drive lock and would not fix the issues! I fittef 3 or 4 different locks before paying the huge cost of a proper one which fixed all teh issues immediately and is still going 5 years down the road,] Drivcers door lock with a key hole is effectively a master cheap replacements were not!!!!