Anyone have a General International 15-250 Sander?

I just bought a new General International 15-250 double drum sander, to be used in finish sanding of hardwood boards. I started by putting 150 grit on the front drum and 220 git on the back drum, and sanding previouslyplaned African Mahogany boards. This wood often exhibits some tearout on the planer, which is why I wanted to finish up on a sander.
After several attempts, I continue to have problems with the sandpaper loostening up on the rear drum, and eventually tearing. I'm using General International's own sandpaper, and I've tried:
-Carefully installing and tightening the sandpaper, per instructions in the manual
-Retightening the sandpaper as soon as it shows signs of loostening.
-Checking the grip of the ends in the clamps that secure it to the drum. The clamps *seem* to grip tight,
-Checking the alignment of the rear drum, and setting the height of the rear drum relative to the front drum, using the procedure given to me by General International tech support. That is, using two 6" flat blocks as a reference, the height of the rear drum with no sandpaper installed should be equal to the height of the front drum with sandpaper installed (ie. the rear drum is one sandpaper's thickness below the front drum).
Anyone have experience with theis sander, and know what's wrong?
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