Last year I used a weatherbeeta bib and still got rubs! The ill fitting weatherbeeta taka freestyle heavyweight didnt help though....funnily enough the day after I bought it I read a review in Your Horse or something where the tester had an ID x with big shoulders, and said it slipped back and rubbed his shoulders. Had I read it 2 days before I would never have bought the bloody thing, though it is great in terms of quality for a horse it fits!
I thought I might try a thicker quilted bib.
Everything seems to slip back until it gets tight on the shoulders though - I wonder if I need to get one that has fixings higher up the neck so that the neck takes the strain not the shoulders, but then i would be worried about him feeling strangled, especially when grazing!