Mask not working

[Maximilian5E6F]

I don't know if I am making a dumb mistake right now, or if the mask is really broken, but while I was following a masking tutorial, I encountered several problems: 

1. The masked area is not getting bigger if I expand the area one frame later. In the original frame, everything works fine.

2. sometimes my "Mask Path" button wont be clickable after I moved a frame forward

I am new to Premiere so excuse me if I am a bit stupid right now, but I hope you can help

[Maximilian5E6F]

I fixed it: You apprently mustn't have the video on V1 even if you just want to mask it with the background