Try updateing the BIOS if you havent already.
Otherwise, it could be a number of things;
Possible faulty MB
Possible faulty chip
Possible faulty PSU
Possible faulty CPU setting (incorrect unlock???)
Possible user error
EDIT: I should note a very curious booting problem of my own that I have found a solution for. Every once in a while when I try and boot, my machine will power on but will not POST. All the settings are correct and everything is plugged in correctly.
The solution I found.........
Unplug the ATX connector from MB, power cycle MB (AKA try and turn on without power), power cycle PSU, plug ATX connector back in and secure it, then power on machine like normal.
Very strange but seems to work everything I get no POST.