My dual link DVI is acting like a single DVI

Hi,
I have a Dell 3007WFP 30" monitor that I have hooked up to my Macbook Air (2012). For some reason, I can only get 1200x800 resolution. In looking through the threads for the same problem, it seems as though this happens to people when they don't have a dual link DVI cable. But I do have one - is there any other reason the Air wouldn't be able to get the monitor to full resolution?

I've looked for new drivers and there aren't any I can find. Any suggestions?

Thanks,
Tom

Yes, I have the apple adapter. I ordered another just in case it was the adapter (not an apple one) but the same thing happened.