That old nut shell.
It is simply down to the type of cable you use and the connection type to the monitor.
Most common Is DVI to DVI, but in order to get the screen to fit the cable used has to be digital Dvi and not analog.
You can tell by looking at the connector end that plugs into the card or monitor.
To the left there should be one small flat pin. Digital DVI.
If there are four pins around it, you have an analog cable, so it may need to be changed.
Check the manual for the display output of each port, and what cable may or must be used.