I am a first-time user of After Effects and currently using the 2015 CC version. I want to apply the roto brush to change the background of the video. The video captures the growth of a plant; however, when i use the roto brush and press play, the same frames are set on a loop. I have removed the span but no success. What can I try to move from one frame to the next so i can use the brush as the plant grows from a seed to a full plant.
Also consider this tutorial which compares the rotobrush workflow to a rotoscoping workflow with mocha (which is also bundled with AE). This may help to decide whether the roto brush is really the right tool for your job:
It's obviously time-lapse footage. Is it in your project as an image sequence? It should be.
And the changes from frame to frame may be so great that you have to tweak Rotobrush frame-by-frame. It's good, but it doesn't work automatically, nor does it work miracles.