Unable to track green screen stock video

#1

Hi all, new to this but wanted to trying tracking a green screen in After Effects 2019 with the use of Mocha.
I have stock video with already a green screen on a desktop PC monitor so I thought this would be easy for Mocha.

However, I have located the screen and put an x-spline

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Screenshot 2018-11-29 at 13.06.03.jpg1392x816 241 KB

but the blue screen location doesn’t track properly the more frames it gos through the more the alignment drops out, I this makes sense? image included

#2

Draw your x spline much, much closer to the edges of the screen and you should get better results. You’re tracking a lot of the background as well as the FG person in that shape. Remember, the shape is where the track is looking, the surface is what the track is doing.

Cheers,
Mary