PC won’t power up

I am assembling a system (identical to three others I have assembled over the past four weeks.

When I plug in the power cord and turn on the back switch to the power supply, the motherboard shows a green LED. When the front cover power switch isturned on the green LED immediately goes out and the power fan pulses.

I had this problem on one of the other systems, but I can’t remember what fixed the problem. I have examined one of the other systems and both systems seem to be jumpered/setup identically.

Any help would be appreciated.

Ralph