Problems with Polygon

Hey there,

I'm dealing with a problem I'm unsure how to solve.
I have an animation with several images that have been lumakeyed into transparent objects using LumaKey and Polygon.
In the pre-render, everything looks normal, but when rendered, there is a clear outline belonging to the box from the polygon. Sometimes it arbitrarily disappears - I am unsure how to make it disappear entirely.
The attached image shows an example of the polygon boxes around the items I have tried to isolate.

Thanks in advance!