This is probably a silly question, but I just got my new 580 ex ii flash today for my 40D. Took about 5 shots outdoor in ETTL mode and the pilot light began to turn off and it would not flash again. I took the batteries out then put them back in again and the light comes back on. Does anyone know why this is happening? Is the flash cooling down because it used too much power outdoor? I took many shots in door and everything is great so far. Thanks for any advice in advance!