I had impingement syndrome a few years back, along with partial locked shoulder on my right side.
After a lot of PT, a few cortisone shots, it eventually got better. My best guess as to the cause for me was my sleeping position. Up until this point my preferred sleeping position was on my stomach with my arms under the pillow. This would put my shoulders in a bad position impingement wise. 6-9 months after I switched to sleeping on my back and/or with my arms at my sides if on my stomach I’m better and have not had problems since. Sleeping position as a possible cause never came up with the doctor or PTs, but after learning more about what causes it and movements/positions that can aggravate the condition I made the change and it seemed to help. I think it was more than a year into the process where I made the sleeping change - I had been doing all the exercises, etc for many months with now improvement before I changed how I slept.
My exact circumstances are likely fairly unique, but something ‘unrelated’ to exercise, etc. could be related to your condition. Looking back for me this seems almost obvious, but it really took me a long time to make that realization, and be determined enough to change a life-long sleeping position - that’s not a fun change to make 
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