You need a different rug prehaps with front pleats so shoulders have more room.Be careful as pleats tend to be in different places.
Had same prob with mine.
You may have to try few makes,borrow some from mates to get idea of fit.
I think a rug with more shoulder room is the way to go. But coconut oil (from a chemist) rubbed onto the shoulders would stop rubbing as well. If rug has already rubbed, the coconut oil will help the hair grow back as well
Whatever the rug on my mare she often gets rubs on the shoulder. So I either put on an under rug with a satiny lining or put a bib on her (also with shiny lining). She's not too bad in the summer, but really suffers in the winter when clipped if I don't put something shiny next to her skin (pampered girlie!!!).