i suggest two things:
1. double-check that the power supply unit works good on a different desktop
2. check that the power switch of your computer case does not get stuck.
one thing you can do is switch the power button and reset button connections to the motherboard. doing this will make the reset button on the computer case to be the new power button.
this happened to me once on my personal desktop. instead of getting a new cpu case, i just swapped the reset and power cable connectors. i suspected that the power button on my cpu case was faulty, so i opted to lose the reset button functionality and make it function as the power instead.