If I don't have them "that tight" they tend to slip off my shoulder (mostly the right shoulder) for some reason. I think I need to move the brace in the back up to prevent this, I've moved it a little ~ twice I think.Ha! Mine on my Nova are the wider ones, too.
For some reason, on the Ultra, they don't offer the wider braces as an option.
I think it's when I let the tension off of them that they will slip. E.g. they are tight, but I bend down and it relaxes the tension on them.
I think if they were pulled so tight that they always have tension on them, then they would probably not slip. But, I don't like them THAT tight.
How far up your back is that?
For reference I am fairly broad shouldered, so they really shouldn't slip (but do)