Nadine Ohara - SF Bay Area/CA , Feb 01, 2006; 10:03 p.m.
Most probable cause is that one usually has to dial in + flash compensation on Canon digitals, even with their own flashes. I'm surprised, because my old 380EX, when used on my 20D, didn't require much plus compensation--maybe + 1/3, and not all the time. Try it and see--dial in a set amount of plus compensation and try various subjects at different distances.
If that isn't it, it could be any of the other issues, although if it were a sync issue, it would either work or not work--same with the hotshoe. Could be recycling...are you giving it time to recycle? Watch the ready light to be sure. Only other thing would be that your subject is too far away for it to expose correctly, but if you tested with close subjects, that probably isn't it. Also, do you have it on auto white balance? Canon's AWB with tungsten is deliberately warm. Also--a very basic thing--check to be sure you haven't dialed in minus flash compensation on the camera.