Have you gone through the Rotobrush training materials? You can add and subtract from the roto. As you work forward, when you get to a frame where the roto fouls up you have to move back until you get a good frame, then move forward to the bad one and carefully subtract or add to the roto before you start letting rotobrush automatically find the edges. With some footage this can be a tedious task. The rotobrush must be accurate before you move on. Once you have the roto complete then you change to the refine edge tool and start again. When everything is done you freeze. If it is a long clip then I would render and replace the footage so you won't have to do it all again if you get a crash or have other problems.
With some footage a simple manual roto or a roto using Mocha is an easier and faster approach. It's hard to tell what the best option would be without looking at the footage. For about half of my roto work I just do it by hand using this technique:
I've been meaning to update this old quickie tutorial for some time now. Maybe I'll work on it tonight.
Thanks for the reply although I think this was a miscommunication on my part. I was trying to say that when I go to add or subtract from the roto in some frames, it does nothing and just stays as inaccurate as before i tried to add or subtract. I did try making the few frames before this happens a bit more accurate anyway with no luck.