Is there any possibility that one of the shoes is stuck on the raised lip of the back plate. Shoe must be brought forward to clear raised lip or the lip will catch the metal mounting of the shoe and interfere with the shoe seating properly. The spring may firmly hold the shoe against the lip interference

Just checking, you appear to have checked most other obvious possibilities.

Good luck

__________________
Martineau Gauda
mgauda@sprintmail.com

96 C280 Mercedes
82 Rolls Royce Silver Spur