To make the rubber grab the belt better, the author uses a tire-traction compound. You may be surprised to discover that your platen isn't flat. C. Get a new one Manufacturers sell replacement cork pads that are cut to size. The author recommends replacing steel backing plates with graphite pads. E. Give it a lube job Front rollers should turn freely and sit parallel to the platen.
Drive belts last a long time, but they do wear out.
When the pad area on the bottom of the sander has a twist or a high spot, one or two corners do all the work, leaving visible valleys in sanded areas. To check a used sander, or a new one before I buy it, I remove the metal friction plate and the soft pad beneath it, which is usually made of cork. I sight down the bottom as if I were sighting the sole of a handplane. The front and rear wheels should lie parallel to the platen, and the platen must be flat. I pay special attention to the diagonals and look for hollows or hills on the platen, which I level with a file.