did you check very carefully to make sure non of the metal standoffs are contacting the backside of the MB in places other than the screw holes? That's been a very common problem area. It's also possible to force start the PS by connecting a small load such as a hard drive, fan, etc., disconnect the MB main power and momentarily short the green pin to a nearby black one. If the PS doesn't turn on, it's likely bad. Another trick is to just connect everything outside of the case. Put the MB on a non-conductive surface, plug in all components and manually short the power on pins on the MB's front panel pin array.