Please fix Set Matte. It fails about 20% of the time and there’s no pattern to when it does.
Can you give us some more detail about what you’re experiencing?
Make sure to have collapse transformations checked on the matte layer.
Roei Tzoref commented
see if this comprehensive set matte tutorial helps:
One thing I've learned is that if you have any scaling on the layer used to make the matte, it's probably not going to work whether you have Stretch Matte to Fit checked or not. Other than that, there are just times it won't work, be off position and I've never been able to detect a pattern when those things happen.