Creality Falcom degree problem.

Hey Guys,
I have a falcon 10W laser. Overall I am very satisfied with it. However, I now have a larger project that requires me to glue several parts together. It consists of 4 parts, all of which are 90 degrees to each other. However, when the parts are cut, the angle is often greater than 90 degrees. Do any of you know why this is happening? The belt tension is checked and good.