Are you still going to PT? - they should be able to locate and explain the snapping and if the shoulder blade is rubbing or hitting anything.
If they can't help you with that then request another PT or PT place with advanced therapists.

Do your PT sessions mainly consist of you doing exercises , or do they do hands on therapies - ultrasound, various massage techniques, rib mobilizations, trigger point, Electric stim etc?

We also have some really good diagrams and anatomy links in our useful sticky thread - http://neurotalk.psychcentral.com/thread84.html