Flash doesn't fire

[w&dy]

My 550ex flash (combination with 10d) doesn't fire in the manual flash mode. I've tried M, Av and Tv priority without any result. It only works when I switch the flash to master function. But the intensity isn't that high on 1/1 as it used to be.
I never had any problems before, any idea what goes wrong?

Thanks,

[mtndew]

Hate to say this but did ya check the batteries????

[w&dy]

Yes I did :).

[scottbergerphoto]

Reset the flash by taking out the batteries while it is turned on and see what happens.