I can't claim to be an expert, so a grain of salt here . . .

have you in fact installed the proper drivers for your motherboard video AND the installed card? standard VGA drivers may not do it.

do both monitors (or at least their graphics cards) show up in device manager? are the drivers installed, no yellow exclamation points?

did you enable both monitors in control panel/ display /settings/ advanced?

and of course, sometimes the hardware just doesn't get along . . .

that's all I know - fj