Elbow range - shoulder position

Hi,

I've recently realised, although probably had this for years, that I can't flex my right elbow fully, actively or passively (there's around 45 degrees more to go) when my upper arm and shoulder blade are held in certain positions. If I pull my shoulder downwards it happens. And if I pull my shoulder back and move the elbow forward to about the position a physio told me I should be when playing the electric guitar sitting down, then it won't bend, but the moment I let the shoulder itself fall forwards (to where the physio said it shouldn't be), then it frees the elbow to flex fully. I can't reproduce this problem on the left side.

Any ideas what's restricting things?