Is it the front edge or edge near the brickwork that is buckling??
If its the front it should be set into a groove in the back of the fascia board
If its the back........I'm afraid you'll have to remove it, make a sort of 'hanger' out of roofing batten (thats the cheapest), one length of horizontal batten with some vertical legs fixed to the sides of the rafters, then refit your soffitt board and tack it up to your new hanger.
Sorry its a bit hard to explain it but its a lot easier to do this when the plastic is being installed the first time, and i am presuming that it is solid upvc board and not cover board (on top of the existing wood?)