600 EX flash not firing at correct time

[Rwak05]

Hello!

I've had two 600 ex flashes that I've had for about 1 1/2 years.  WIth one of them, I've had no problems.  With the second one, I have problems with.  It fires, but doesn't sync up with the shutter speed.  Almost like it's firing just before the image is captures.  When I test it out, the flash fires and then directly after I hear the shutter open and close. With my working flash, this isn't happening so I don't think it's a camera setting.  I've reset the flash back to factory settings to make sure I didn't inadvernently change a custom function setting, etc.  Does anyone have idea if I might be overlooking something or what I might do to remedy this?  Thought I would start here before sending it in for repair.

Thank you!

[Rwak05]

I've come to realize that this happens only when using ETTL wirelessly and occasionally on camera. Manual works every time. I've re-read the manual, checked lots of settings, reset settings on flash and camera and still can't figure this out. I would think that it possibly could be a flash unit problem, but it happens wirelessly with either flash. I've got to be missing something but for the life of me I can't figure out what it could be.