Hi, my name is Andrés and I am Colombian, therefore I apologize for my English.
Generally this problem is about different tolerances between the files, its check the parameter of "Accuracy"(File - Prepare - Model Properties) in the inherit reference model and the file of machining, they must be same otherwise change the machining file.
Sometimes the reference model has bad surfaces or with errors, repair the model before of entering it in the machining file.
Regards.
