Sound to me as though you have been trying a little to much to soon. Why are you so worried on getting more external rotation so soon? How is your flexion and abduction? Shoulder take a long...long... time to heal and restore function and in my opinion should not be pushed, especially following a fracture.

Do you have any active external rotation strength from full internal rotation back to neutral? i.e. are the external rotators intact. Is your neuro assessment normal? Lot's to ask really. Did the surgeon get more passive external rotation on the table etc.

Also how did you injure it? Any x-ray or Wikipedia reference-linkMRI images you can post?