Roto Brush refine edge is working fine until one frame

The rotobrush + refine edge tool has worked as it's supposed to through all of the frames until the 31st frame, when suddenly, the refine edge brush seems to do nothing; the head of the person is nicely isolated with the slight feather around the edges then suddenly is harsh and blunt in one frame.  I've restarted my computer, the program, everythhing, but the refine edge of the rotobrush tool refuses to work on this one frame. 

How on earth do I make the tool work on the frame?  It's driving me insane!