Mini-ITX fan starts then stops when using pico-PSU.. why??

[nappyjim]

Gigabyte AORUS B450 mini itx motherboard
Ryzen 3 3200G CPU
Corsair Vengeance DDR4 3600Mhz 8GB
12V 10A 120W AC-DC converter
150w & 250w PICO PSU

When I start PC, CPU fan will spin, then stop. Thought 150W PSU too small maybe, got a 250W, same thing. Connect my full size tower PSU to the board, powers up fine. But I dont get it; I have ZERO connected to the motherboard other than the CPU, CPU fan, and the RAM. What gives??

 

[confusis]

Is there a short circuit somewhere in the wiring - that seems to be the behaviour for that.