Some checks

You need to check that all required power connections are made and actually the PSU can handle from whatever power leg being used(swap leads if possible). That the bios reflects that it can access other than on-board video. Any other bios setting that may allow easy of access for the video card. At what bios level is it at; check for current bios update releases. Give it more time during the boot-up as better video cards do some setup and/or boot it's parameters. These are the basics for now but if you can check the video card on another PC, what are the results. Sometimes, you have a bad video card or it's too demanding.

tada -----Willy happy