#1. "RE: Picture underexposed" In response to Reply # 0
I've read the comments in the other forum and the only other suggestion I can make would be to adjust the SB80 head downward. White card out is OK. I would position the head so the light bouncing from the ceiling is reflected toward the subject (about 1/3 of the way between flash and subject). At 90 degrees you are effectively bouncing the light straight down onto the camera again. As suggested the white background may be effecting the exposure so increase flash compensation about 1/3 or 2/3 stops.
Also what aperture were you using? If you were using a small aperture that may also be a cause. If so open up.