That's logical ..
Some old monitors cann't be seen by video cards so the video card won't work & the motherboard will swich to the onboard vga automaticlly ..
however, try to disable the onboard vga controller from the bios .. this should force the using of any video card installed ..
NOTE: If it doesn't work you will need to rest the bios to enabel the onboard vga controller !
I'll think in more options ..
regards