I just bought my first 580II, after having been using a Sigma 530 Super for a couple years. I never had any e-ttl problems, or issues where the flash decided to not fire.
So I unboxed the flash, put in brand spanking new batteries, put my XTi on Av at about 5.6, and nothing. Played around, stared at it a bit, clicked the shutter a few times. Still nothing. Then randomly, SOMETHING! Then it worked just fine in e-ttl. Then out of nowhere, nothing.
It went on like this for a couple hours, where it would fire sometimes, and then not. I'm normally really good at fixing stuff like this, and also good about not asking stupid questions without research. I've found nothing that really explains what's going on here. I apologize for being so vague, but this phenomenon is just as random as it sounds.
I've also noticed that it repeatedly switches between e-ttl and ttl on the back of the flash, completely at random (from what I can tell).
Can someone please give me a heads up as to what's going on here? Is it user error? Flash error? Body error?
Oh, and I'm not sure if this is relevant, but my pop-up flash stopped popping up about a year ago, but it doesnt bother me at all because I only used the sigma anyway.