+1 exposure compensation is irrelevant. You need to dial in +1 flash exposure compensation, which is entirely separate and independent. Hold down the rearmost of the 3 bottons on the top left of the 1Ds-II, and rotate the rear dial clockwise.

I have been using my 20D for a back up since I got my 1Ds Mk II. When using a lumequest bounce panel I consistently get approx. 1 stop under exposure. When I use a +1 exposure compensation it doesn't change. I know the flash can put out enough juice as the recycle time is very fast. The problem is much less pronounced when I use direct flash.

Dr. Gary