5dMK2 Bounce Flash Headache

Hey folks looking for a bit of help here.

I was shooting a wedding earlier today (5DmK2 with 28-70 f/2.8L) and my flash (550 ex) was grossly underexposing the scene. I used my partner's camera (Nikon d3 with SB-800) and with the same exact settings and his camera was exposing the room perfectly every single time. The room had an 8 foot drop ceiling (white). I have shot in rooms with 20 foot ceilings and had better results that what I was getting today. Very frustrated.

Both cams were set to Manual, ISO 640, f/5.6, 1/125 sec. Any ideas? Could my flash be the problem/ dying (several years old)? I tried all of the metering modes and found that none were really affecting the image. I tried using some FEC but even that didn't seem to be producing an effective amount of light. Any ideas?

Is the Nikon flash system THAT much better than Canon's? I have shot about 50 weddings and never had this much of an issue.
--
Tim Devine,

owner of The Magic in Pixels.com, a site by Disney Theme Park Photographers for Disney Theme Park Photographers.