GPU shows as working, but won't output signal

Hey everyone, I’m currently putting together a retro build—strictly a nostalgia trip meant for running Windows XP and those classic games from the late 90s and early 2000s. My goal is to scavenge parts from a few old machines to create one really high-end rig for its era, but I’ve hit a massive wall with an AGP graphics card. Here’s the weird part: everything seems to be working perfectly on paper. The GPU runs flawlessly in its original machine, and the AGP slot on my target motherboard isn't broken—I've tested it with other cards and they work just fine. Yet, when I try to pair this specific card with this specific board, nothing happens. No POST, no nothing. I've already flashed the BIOS to the latest version available, too. Has anyone else run into this kind of compatibility headache? Any ideas on what might be causing this standoff?