Might be time to order some new clamps and set screws. You can order them from
here
http://www.dewaltservicenet.com/ServiceNet/logon.asp if you have no place to get parts locally. You have to register first to access the site. Part Numbers are Knob Assembly/Clamp 286303-00 and Set Screws 286301-00. You need two of each (one each for the upper clamp and lower clamp).
When I ordered some I got a couple of sets so I would have some on hand.