Hi Paula,
We had similar surgery and I also could not lift my arm up on the side of the mandibulectomy.
I did PT for quite a while which may have helped but in the end the therapist told me he could not do any more and it would come good in it's own time.
Sure enough one day I realised while showering that I could raise my arm to wash my hair shocked
I have a theory and may have even been told that during the very long surgery my arm was placed up and behind my head.
Best of luck with your ongoing recovery.
Gabriele

History Leukoplakia bx 8/2006 SCC floor mouth T3N0M0- Verrucous Carcinoma.
14 hour 0p SCC-Right ND/excision/marginal mandibulectomy 9/2006, 4 teeth removed, flap from wrist, trach-ng 6 days- no chemo/rad.
6 ops and debulking (flap/tongue join) + bx's 2006-2012.
bx Jan 2012 Hyperkeratosis-Epithelial Dysplasia
24cm GIST tumour removed 8/2013. Indefinite Oral Chemo.

1/31/16 passed away peacefully surrounded by family