580 EX II firing (with light) but picture still comes out dark

[ShaimaPhoto]

Hello,

I bought my 580 EX II back in December 2007. I have been using it mainly in summers during wedding season, always on ETTL mode.

Last week, something weird happened. It fires (I see the light), but the picture comes out dark. I have checked advice by googilng the problem and tried whatever was recommended (cleaning the connectors, resetting all the flash functions through the camera, etc).

Also, I have a second 580 EX II which is working with the same camera and the same settings.

I have NO idea what this problem is or how to fix it.

Any ideas??

Thanks!
Shaima