If its a physical adapter the card and monitor have no idea anything is being converted, so it's not a setting issue. Have you tested the monitor that isn't recognized to make sure it works? After that I'd check the cable, then the converter itself, then I'd start looking at video card drivers? Has this card worked with dual monitors before? If not, make sure drivers are updated and or reinstalled to the latest version.
If all that fails you may have a bad secondary output on that card.
There are several aspects:
1. Are output to monitor configured properly?
2. If DVI have no analog signal, the there is no chance to use VGA monitor.
3. I do not recommend using VGA in combination with digital monitor. Namely when there is a need to use high resolution. Lower quality of picture is recognized easily.
4. There are special graphical cards for multiple monitors.
Regards
Milos
If the port is DVI-D there is no analogue signal. It must be DVI-I.
A second motherboard port must be enabled in the motherboard BIOS. Some motherboards allow only one video port at a time to be used, and may not accept converter plugs. Check your motherboard manual.