You didn't say how old or how many hours your head had on it, but many years ago I had a CII that would act like that in wheat. What I found was that the flighting on the conveyer was worn and rounded off, thus it wouldn't grab the crop and pull it under. A quick and dirty fix is to take a hammer and chisel and go along the flighting and make a sharp spot about every 6 inches to grab the crop. The better way is to run a small bead of weld along the top and grind it off square and sharp again.