An atx mobo switches on electrically. When you push the on button on the 
front panel the switch grounds a point on the mobo which electrically turns 
the power supply on. Check the wiring from the mobo to the on switch and the 
wire from the switch to make sure it is making a good ground. Sometimes the 
grd is made at the switch although most boards have a grd terminal on the 
mobo. Chk your manual.
Also, you might check to make sure the +5v connection to the mobo is okay 
and is +5v.. This is the voltage used to switch the PS on.

HTH
John Dinges 

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