When you finished using the rotobrush, did you press the "freeze" button in the layer window?
AfterEffects CS5: RotoBrush
Johnny Cuevas, Editor
Thinkck.com
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.