Do you mean the seat isn't locking in position ?
If so 3 reasons that I can think of.
Cable outer is pulled too tight - ie ends not seated correctly
Trim clips / JIS shoulder screws are not holding the rear cowl properly in position
Something is jamming the latch
This might help:
Pictures and notes of the rear cowl, grab rail and how the seat release mechanism is fitted and secured. Nothing difficult here, but if you have never seen the release cable and don't know how it should be put back, the pictures may help. Here's the photo of what we are talking about - under...
www.st-owners.com
If you mean that the rear seat is locking onto the loop under the seat - then any of the above - plus the loop is bent - usually caused by tring to slam the seat down when it didn't want to latch.
It can be bent in such a way that the loop doesn't align with the slot with the latch, or the bottom par of the U loop has a kink making it more like a flatted W.