[Les Meehan]

Brian, slightly confused here by your info. You say the shots are UNDER but that the histogram is cut off at the HIGH end? Isn't that a contradiction? If the histogram is pushed up to the high (white) end then the shots must be OVER exposed, no?

I don't know the equipment you used but it suggests to me that there simply wasn't enough light output for the ISO and/or f/No being used. The diffuser will have reduced output by at least one stop if not more, did you compensate for this? Was everything on 'auto' and so possibly picking up bright reflected light from shiny environment, which might cause the flash to cut-off too quick.

Can you supply us with an example to look at together with relevant technical info?