AX3SP Pro mobo: no boot/just standby, that's it.

Got this desktop from my brother-in-law just 2 wks. ago, started up fine at his place.

I cannot get it to start up-- thought it might be one of the jumpers for power switch (power sw), but they're all set correctly.

AOpen Power Supply (300W) works fine, I tested it on another machine, but this one is supposed to be an upgrade from my current desktop, so I'm not going back to the old one.

Only time that the power supply fan works on this board is when I put the P1 connector quarter- to half-way into the board. The Boot LED also lights up.
But if I push the P1 connector all the way in, it doesn't boot, the fan doesn't turn, and the only thing that is on is the Standby LED.

I removed all peripherals for the time being to keep testing, but I'm at my wits end.