try adding the power switch lead to your setup, if this does not work try pluging in a 3.5 inch hard drive, it sounds like their isnt enough load on the 12v line for the power suppy to boot.
Got a m2-atx power supply , connected it up off of the 12v rail on my PC power supply.. basically as soon as i turn on the Power supplys main pc power the LED just flashes and goes off.
I havnt got nothing connected to it ... Just the m2-atx .. to the mainboard and the p4 connector .. No switch wire connected.. I assume the LED is ment to stay lit ?
When i use a normal PC power suppy on it , its fine
any help will be apreciated