Dear Steve:
It seems  that  the  mounting media you used retracts with the time. This 
kind of artifact i have  seen working with sinthetic mounting media that has
no tolerance for heat and, in other cases, with aging. Try to find an
adecquate medium, I proved  with  success  Clarion  from Biomeda, and I 
have  slides mounted with it  that have no changes -no bubbles,no retraction
no cristalization- after many years.
Good luck,Carlos