I used to have the same problem. I used a shoulder sleezy, and it just rubbed her in a different spot. But there are 2 different kinds of sleezies. One goes in between their front legs and buckles around their withers, and the other just goes across their chest. (hard to describe) I used the latter one, and it rubbed her. You can put show sheen on their shoulders so the blanket can move easily across the shoulders, I did this, and it helped. You may have to get a different blanket bc like said before, every blanket fits differently. I unfortunetly ended up buying a different blanket!!
We call them anti rub vests here in NZ - I have one on my horse in winter (A padded cotton one and a slinky thin one in summer when she has her fly sheet on... No more rubs ;)
Thanks again everyone! I've changed his blanket to one that fits better, and put show sheen on his shoulders, it seems to be helping. Getting him a shoulder cover thing as soon as I can.