In hydrodilation, the glenohumeral capsule is filled with an infusion to "stretch" the capsule, in a hope of freeing the shoulder. Frozen shoulder is poorly disgnosed (over diagnosed) and is therefore poorly discussed. As is the cause!
Arthroscopic investigation shows a very red, angry and irritated synovium. Surgery and clean up is successful in returning movement soon rather than later. Other recommend doing nothing and it will self heal in 1-2 yrs, the later often being close to the truth.
I would look for a cause (perhaps a ruptured rotator cuff tendon or even a bankart lesion perhaps from sleeping on an unstable shoulder). Then address the issues and perhaps clean up the joint if you want a quick result.
Conservative management with connective tissue release, particularly to subscapularis, inhibition of upper traps with taping, rotator cuff balancing exercises etc all assist in the rehab - but if it isn't happening steadily I would look to arthroscopy from a good shoulder guy.