Did you use the ".inf" file that came with the monitor when you installed it?
Did it come with one?
Are you running Windows?
What version?
Is your monitor correctly identified under "system devices"?
I had this problem when I got my last monitor.
They had actually put an incorrect setting in the ".inf" file.
I had to edit the file, (using my monitor's manual for reference)
to put in the correct setting.