Adobe After Effects Rotoscope Bug
So, I was rotoscoping to make a green screen video, and the rotoscope was working great. Then, I went to the next frame, and the selection wasn't right at all, so I made the brush big to remove all of it. Then I started doing it normally again, but it was like I was just painting on the frame. It didn't select the colors or ANYTHING, just like I was drawing on the frame. I have almost always had this problem, no matter how much I update my software or computer. Please help!
My computer is Windows 10 64-bit and version of After Effects is After Effects CC 2017