The bed's not level. Get a measuring stick, like a digital caliper, and go to the back right and measure the distance to the bed. Then go to the back left, it should be the same, Repeat with the front right and left. All should be the same. If any are off, you'll need to raise or lower just that end. Do that by either loosening the belt and turning that screw, or if yours will allow, loosen the sprocket on the one that needs adjusting and turn the screw.
