Rotobrush only propagates in one direction. You have to increase the number of frames that you want to analyze, check the progress, make corrections, and then when you are finished with the entire clip you freeze the Rotobrush.
I would strongly suggest that you type "Rotobrush" in the search help field at the top right corner of AE and study the help file and look at some tutorials because I don't know anyone that has ever figured out how to use Rotobrush without some study.
Roto brush is also a resource hog. Once you get a good matte made, it is a real good idea to render a digital intermediate with an alpha channel to use and the rest of your production. I always do this if the shot is longer than the 5 or 10 seconds.
If you are talking about removing the background and 30 or 40 seconds of video I would seriously consider cutting it up into pieces 10 seconds long.
are you suggesting that you want a one-click solution to removing the background? that could only be the case if the subject was shot on a green screen with perfect lighting. how long is your video? while RotoBrush is Magic, it needs a skilled operator to know the pitfalls - when to insist on a part of the shot, and when to find another way. also use the proper workflow because as Rick said it is a very resource intensive effect. more about the proper way to do it is suggested here:
notice how long this manual is - that is because many look for a magic solution and get disappointing because they are not following the workflow. take some time and experiment.
to address what Rick wrote:
Rotobrush only propagates in one direction. You have to increase the number of frames that you want to analyze,
to be more accurate Rotobrush propagates from the base-frame outwards. it propagates forward and backwards in time but it will do each at a time depends on where you set your corrective strokes - meaning you should select the best base-frame you can and more often then not - it is not the first frame or last. you get 20 frames to each direction by default - this is called a brush span and if you set a corrective stroke near the end of the span you will get more. this is all done to reserve resources and not render what is not a corrected matte.
we will know more how to help you when we examine your video.