Was Master/Slave switch on the 550EX set to slave mode before you switched it to Manual mode?
The person I bought the ST-E2 said he could not get them to fire in manual. The test I did was very simple (did not involve a camera). I just sat the flashes on a table and made sure the ST-E2 made them fire.
I just got some umbrellas and will be doing some real tests tomorrow.


