help with rotobrush

Hello., i have video of an interview. The subject is standing and the background is bush. The subject only moves at times hands or sometimes the arms. Now i want to remove the background. With resolve magic mask is almost perfect, but for example, between the finger, when they open i see details of the background. Also the arms..sometimes are well along the body..sometimes they slightly move aside and then pieces of background come out. Now with roto brush i want to clean those things

So i move the playhead in layer panel view, until i find a part where to add or remove somethings. But i can't. Nothing happen. Wheter i add or remove nothing happen

How do i do that?

Thank you

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