If your remove isn’t fully removing it is usually because the background track is incomplete, bad, or trying to track multiple planes at once with lots of parallax.
It could also be because there are not enough pixels moving behind the object you are trying to remove for there to be an automatic replacement and you might have to create a “hand-painted” cleanplate to give Mocha something to replace the object with.
If the Remove isn’t removing at all, the background may not be tracked, or it is referencing frames that are out of range or stationary.
Ask on the forum with your specific project example for more help: Mocha Forum