Roto brush problem (Urgent pls help)

I am trying to remove the background from my video using Roto Brush. This is my first time. To reach till 25th second it took me 8 hours. If I make a correction, it again starts from the beginning and takes a lot of time. But on reaching the 25th second, it stopped, it is not going further and just start from the beginning. What to do? Please help.
