This isn't the definite solution, but it's just something to check for. We had a monitor that had its screen resolution off center... majorly off center (so much so that it could not be corrected fully via adjusting the horizontal position on the monitor settings).
I'd read some stuff online about it being a VGA connection (vs. DVI/DisplayPort/HDMI) not supporting that monitor's full resolution, so I tried switching to a monitor that I knew worked with VGA. Same issue.
Turns out it was a faulty mini-DisplayPort adapter. So, that won't be the solution every time, but that's something to definitely look into, if you experience this problem.