I was beginning to wonder if I would be able to fix this problem without re-creating the detail. I think it happens with view erase and resume based on some drawing representations I have been using to help with drawing performance.
Anyway, I used VIEW - DRAWING DISPLAY - DRAWING VIEW VISIBILITY and i erased the view and resumed it again and the label came back and hasn't disappeared since.
I've had intermittent problems with this since we moved to WF4 but this was the first time I couldn't get the label to come back with a regen view.
Thanks