AFTER EFFECTS ROTO BRUSH - DOESN'T HIGHLIGHT SUBJECT BUT THE WHOLE FRAME

So, as the subject mentioned. I try to roto my footage but it goes highlight the whole frame. Sometimes, it works properly sometimes it becomes like this, and I can't figure out whats the problem. Any ideas what's wrong with this? I don't think need a picture to explain this. Just imagine you want to highlight the subject but it roto the whole screen.