Marilyn,
I've had several Vivitar 283s experience that "problem". Some were mine and
some were given to me once this failure ocurred. There is a company that makes
metal replacement mounts for the 283 and I bought a couple and have functional
repaired units. Am not sure if anyone makes them for Canon or other flashes.
One reason that the plastic shoe mounts are installed on the flashes is that
they are cheap. Another is that if the connection were really strong then the
camera body around the mirror housing might break if a strong enough force is
applied to the connection such as by twisting of the flash for some reason. So
to avoid catastrophic failure the weak link breaks instead. Small consolation I
know.
Andy
Andrew Davidhazy, School of Photographic Arts and Sciences/RIT
andpph@rit.edu http://www.rit.edu/~andpph
