From what I understand the accessory nerve in the shoulder area is bruised or irritated or damaged by the surgery and it can be hard to lift the arm. Crawling your arm up a wall is a great exercise for this. Just go a little bit further each day. I don't think you can go wrong with the surgeon's exercises because he/she will know which structures have been interfered with!

1996, ovarian cancer surgery + cisplatin and taxol.
September, 2007, SCC of left lateral tongue. Excision.
October, 2009 recurrence in scar tissue, T1NOMO. Free flap surgery from left wrist - neck dissection. 63 year old New Zealander. No chemo, no RT.
February, 2014. New primary in left buccal mucosa. Marginal mandibulectomy, neck dissection, right arm free forearm flap. T1N0M0 but third occurrence and some areas of concern: RT started 8 April and finished 19 May.