Problem rotobrush

I am not sure why you are using Rotobrush on Green Screen footage. The boundary overlay is pink, so you are not looking at the matte overlay. I see a bit of expanding going on, but this is probably normal.

Rotobrush is a tool of last resort when you have to separate an object from the background. It does a pretty good job if the object you are trying to isolate has good edge detail, minimal motion blur, and different color and luminance values than the background. Using Rotobrush on Greenscreen footage is about as efficient as hammering nails with a screwdriver. 

Ctrl/Cmnd + Left or Right Arrow might be a better way to move the CTI one frame to the left or right. 

Okay, your video suddenly appeared (probably a forum glitch) and now everything is clear. Rotobrush does not work on images that have no "real" motion. Your strawberries are merely rotating and there is nothing Rotobrush could latch on. You need to use masks or simply key out the green background, which likely is the intended procedure, anyway.

Mylenium

There's nothing to see here. You need to provide an actual link to your video or embed it here (as long as it fits the 40 MB limit) plus of course even with the video you need to provide exact info on your system, effect settings and whatever else may be relevant.

Mylenium