Geoff I know this is probably going for the absolutely obvious but is a problem I've struggled with for years and it's linked to a neck problem, probably limited mobility in the cervical spine.... As you come close to "full draw" there can be a very slight tendency to push the neck forward without tipping forward - effectively shortens the draw and then you end up further down the side of the face to get the line.
I know a couple of others of us with the same stiff (weak?) neck problem. You have to watch closely for it because it is only the slightest push forward.
Other option (spectacle wearers beware...) - too much head rotation...you mentioned this I think - head too square...
Pain in the neck, mate!