PSU Clicks once then turns off

I’m building a new pc & I got most of my parts today, figured I’d check for faulty components. Everything built and done, but when I jump my mobo my PSU clicks once and shuts down. I’m pretty sure it’s the PSU as I used an old EVGA PSU jumper on it and the issue still persisted. Any ideas or is it RMA time? It’s an FSP Flex atx 400w Gold