Computer Power-Up

The last 3 computers that have come into my shop have given me the same problem — no matter what I do, I can not get the system to power up. I have done the following to all 3 systems:

1. Replaced the MB and ALL componets inside (HDD,Optical, FDD, Fans, Cables)
2. I have used different power supplies in each. (450W – 600W)

In each case, I get LED lights on the MB, the fans on the CPU are spinning but nothing else! (No POST, no beeps, nothing) The only thing that I really have not changed is the chassis. (SIDE NOTE – I have changed parts from one chassis to another and updated the BOIS but still the same results.)

In 13 years of IT, this is a first for me. Any advice?