I might be mistaken, but I don't think you can use a drum sander to flatten
boards, just like you can't use a planer to flatten boards. The rollers
will compress the board flat, sand it, then the board springs back to it's
First thing to do is confirm that the board was indeed dead flat before you
put it through the sander. If not, try taking several very, very light
passes with the concave side down. Then flip the board over and repeat.
With patience, you can pretty much flatten a board with this technique, and
enough extra wood thickness...
What model sander is it? I have the Delta 18-36 and do not have this