White glue shrinks when it dries. Maybe you could use a white glue solution
on the back and see if it shrinks around the pucker and flattens it out.
There are wooden pieces in the shape of a triangle that can be pushed into
the corners to tighten the stretch on a canvas. See your local art store for
these.
Ken Schwab
Art teacher, Leigh high school, San Jose, CA
website: Leigh Art