What is the reason for the low linear power of RDV6555G controller?

When I was using the RDV6555G control system for machining, the power output of the equipment was normal when running on short straight lines, and the machining effect was stable without any obvious abnormalities. However, when the processing path becomes a longer straight line, the actual output power is significantly lighter and the material cannot be cut through. I tested with the same materials separately, but the problem still persists. Who can give me advice? Thanks.