Having owned several of the Sonor pancake models, I believe that the situation you describe is a typical issue encountered by owners of these drums. The problem might be set right by trying the following procedure:
Taking a sharp razor knife of the type used by hobbyists, cut the outer aluminum counterhoop rings off of two used plastic drumheads.
Put your drumheads on the pancake snare as you would normally, and lay one of the trimmed aluminum rings down on the head, creating a doubly thick aluminum counterhoop.
Repeat this procedure for the other side.
Then lay the metal hoops down over each head, insert your tension rods, and re-tension the drum.
The extra layer buffer should hopefully give you enough "elbow room" to properly tension and set up your drum without the metal hoops meeting each other and hiding the shell!
The only other option I can suggest is using different heads such as those made by Attack or Ludwig - with a very narrow crimped style aluminum counterhoop securing the mylar.
Remo heads and some others have a deep channeled counterhoop. These seat themselves deep over the shell, increasing the chances of the problem occurring.
Best of luck!