Lets troubleshoot it. You are using a sled, right? And does it slide smoothly? Do you have enough infeed/outfeed support? Are you able to feed it at a consistent rate past the drum? Do you have dust collection at the drum head, or are you getting dust buildup on the drum or table? Have you checked to make sure your table is not being deflected by the pressure? Are the lams coming out smooth or scalloped? Are they even thickness from side to side?
Are you taking light passes, or trying to hog off material? What grit and type of paper are you using and is it well adhered to the drum? Is your adjustment mechanism solid and have you checked to make sure it does not move during operation?