[joshman]

i can almost guarantee it's the pre-flash. i have the sigma flash as well and i've had this come up a few times. as bes ti can under stand it, when the on camera flash fires, it actually fires twice, once to gain aproper exposure value, and again to actually take the photo. the speed with which it fires is too fast for use to determine that it's two flashes. anyway, teh first two of these flashes sets off the slave flashes.

so when it come to actually taking the picture (the second flash), the slave flashes don't respond cause they've already fired.

and that's why you photos are black/underexposed, etc.

the easy way around it is to manually do the pre-flash. once focus is obtained, press the exposure lock button (*), this should fire the pre-flash. wait for the flashes to recharged then fully depress the shutter button
hey presto, a picture!