The snapping scapula is related to a bursa under the scapula - often the medial edge called scapulothoracic bursitis.
A good free online review including non-operative treatment is here:
Manske, Robert C., Michael P. Reiman, and Mark L. Stovak. "Nonoperative and operative management of snapping scapula." The American journal of sports medicine 32.6 (2004): 1554-1565.
http://www.drskedros.com/wp-content/...ng-Scapula.pdf

causes include any boney spicules (osteophytes) and "anterior angulation of the medial scapula in the axial plane"
Treatments include:
physical therapy
anti-inflammatory medications
activity modification (see above article)
steroid injection

Recent article on operative management:
Menge, Travis J., et al. "Arthroscopic treatment of snapping scapula syndrome: outcomes at minimum of 2 years." Arthroscopy: The Journal of Arthroscopic & Related Surgery 33.4 (2017): 726-732.
Arthroscopic Treatment of Snapping Scapula Syndrome: Outcomes at Minimum of 2*Years - ScienceDirect

includes arthroscopic bursectomy of bursal hypertrophy causes significant improvement
revision is a possiblility
This is fairly new concept so you need surgery, you may want to contact the clinic for suggestions of where to go for arthroscopy (or go there):
Contact info in 2016 was
Peter J. Millett, M.D., M.Sc., Center for Outcomes-based Orthopaedic Research, Steadman Philippon Research Institute, 181 W Meadow Dr, Ste 1000, Vail, CO 81657, U.S.A. E-mail:[email protected]