Rotobrush 3.0: i can only modifiy the first frame

Hello, im new to AE. I try to use rotobrush but When i pause on the second frame im not able to modify my rotobrush like on the first one and it doesnt change if i hit the button "freeze". Does anyone has an idea why i can't? everything look the same in the second frame except that the outline doesn't change. Im really stuck