i just had to install a replacement lock mechanism because of the same thing. the weak plastic bracket the bolt goes through at the rear of the bit housing the barrel had snapped off at some point in the past and had been bodge repaired. result was that when the seat was pushed down to lock, the whole thing moved down and it was unable to latch.
worth checking that out. its not hard to get the lock mech off but you do have to remove the luggage carrier (rack).