10/20" Drum Sander Alignment Problem

I bought a 10"/20" Powermatic drum sander from Woodcraft last year. It seems to have an alignment problem: when I sand a board that's about 8" wide, it comes out slighly thicker on one edge than the other - about .010" thicker on the inboard side (the side closest to the motor). How do I adjust this thing so that the spacing between the drum and the bed underneath is uniform all the way across?

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