Check your cable's version if you're using the latest MBP. Also, check if your output device can handle it. My dell u3011 can't handle even 2560x1600 through HD

Check your cable's version if you're using the latest MBP. Also, check if your output device can handle it. My dell u3011 can't handle even 2560x1600 through HDMI (which it has 2 for some reason).