When I do the pin trick it works.
What you're doing is simulating the motherboard sending the PS_ON# or Power On signal to the PSU. The motherboard will only send out this signal to the PSU if it has determined that all of the PSU's voltages are within ATX12V specs.
Since the motherboard isn't sending this PS_ON# or Power On signal to the PSU then there must be a problem with the PSU.
Borrow and try a different/better PSU to troubleshoot with.