I have a Supermax 37-2 Dual Drum Sander. I cannot seem to get the conveyor belt to track straight. I adjust the back roller to be exactly parallel to the front roller and the conveyor belt will run straight until I start to sand some wood - I am sanding 2" wide poplar with 120 and 150 and very light take off. After I sand maybe 100 feet of the wood the conveyor belt will have shifted to the right side. I have tried to put more tension on the right and then the left but the belt always climbs to the right. What am I doing wrong. Help please