Pictuees too dark on 50D with 580EX - please help

Hi,

I am no where near a professional - I simply want to use my 580EX on my 50D in a fully automatic mode.  My expectations are that I put the flash on the camera, set it to ETTL like is says in the manual, set the camera to P and fire away.

But all I am getting is a really, really dark pic - like it is heaps underexposed. ie you can barely see anything.

I put the flash on my old rebel - and all works fine. Am i missing something?

I dont want to use M or fiddle with the use exposure settings - just it to be as aumormatic as possible.

Kind reagrds,

Mil