I would suggest to remove the fasteners that hold the edge of the surround, try to reposition the edge and bottom of the surround panel so it fits. I would guess that age has pushed the tub and surround a bit out of square and that is your problem with the fitment. It may also be that the supporting floor material that supports the tub has had some water infiltration and has swelled. That would be enough to cause the small amount of bad fitment.
I would glue the surround panel to the wall and put new fasteners in the holes of the surround edge. The screws would basically serve as hole fillers and to hold the surround in place until the glue sets.