Your trying to trim off a small amount of material, right? If so this is a jointer operation and needs to have the outfeed of the fence set up even with the flush trim bit and the infeed needs to be set up to a depth which matches the amount you want to trim.
Check out the "The principle of the Jointer Fence".... http://www.routerforums.com/showthread.php?t=242
Check out the "The principle of the Jointer Fence".... http://www.routerforums.com/showthread.php?t=242