Not sure..you wrote dial-a-matic...that's header height control. I think you are refering to the big black knob on the console to adjust the reel speed...RightIJ
If your talking about the big black knob. I had the same problem and the hose clamps that attach the knob to the valve needed replacing and tightening. Took about 30 minutes once you remove the flat balck plate from around the gearshift, throttle and cylinder speed levers.