I've landed to this thread looking for a solution to the same problem and I'm glad I can certainly provide some good information.
I had or still have the same "flash firing but still dark picture problem". But my situation was kind of different and couldn't read anything like it either here or anywhere else, here's the thing.
First I'll describe my equipment:
- Canon 60D
- 1 Canon 580EX II
- 3 YN565EX
I used to shoot with the pop up flash and have all my external flashes as slaves, responding to the pop up light beam and it was like many people says, even the flashes fires, the picture looked like there where no flash, now I could get it to work, noticing that it will only do if I keep the aperture in the minimum available by the lens, if I step it up just by one stop, I had no flash on my picture even they're actually firing up.
Now I'm pretty sure about this because I get to photograph the flash itself and I saw with my bear eyes that it fired, but in the picture appears completely off.
Even like this, sometimes it keep giving me the same problem, I was frustrated, tired of looking the net and not finding a solution. I was pretty convinced that it was a sync problem, for some reason the flash and shutter were unable to match. The problem is that I couldn't find a way to manually adjust the shutter speed on the flashes, and I stopped looking how to because I found a great alternative.
I had a Yongnuo RF-603 II unopened since bought it a week ago, and finally decided to try them and VOILA! It ALWAYS works! Now read me very well here, it always works! No matter which aperture, ISO, or shutter speed I choose, it always works! I was even surprised because it let me shoot with a shutter speed above 250 which Y'all know is the limit for flash use, and it works! I have FULL control of the image! I'm pretty sure about it, I got tired making tests, it worked always! I'm so excited I'm getting two more for my other flashes right away!
Now read me well, this is exactly why I believe that the previous issue is due to a flash firing too soon or late, you'll resolve the thing either you get them synchronized or you get a Yongnuo RF-603 II like I did.
Hope you guys the best and problems solved.
P.D: Even I'm voting for a sync issue, I'm intrigued that it only worked if I kept the APERTURE at minimum (no a single stop up), not the SHUTTER SPEED. Weird huh?