Computer with 2x monitors not displaying full res on second monitor

[tricky]

A colleague has a machine with a GTX670 card in it. He has been running with 1x Dell display at 2560*1440 resolution. This is the native res of the screen.

He now has a second identical monitor, but he can't get the second monitor running at the full resolution. We have tried countless cables (HDMI, DVI) but cannot get the second monitor running at full res.

Any ideas why this might be?