Roto Brush Problem

Hello everyone

I am trying to roto a 1:05 clip with roto brush in AE CC. When I get 34 seconds in and click to the next frame the mask disappears. If I go back one frame it’s there then forward again and it disappears as if there is no roto. I have 32 gig ram and a large cache. What am I doing wrong? How do I roto one person out of a clip that long?

Thanks in advance for all your help.