"Cannot find another display"--but the screen is duplicated?

Good afternoon,

I'm trying to hook an XPS 15-9560 laptop to a Dell Ultrasharp monitor, using an HDMI cable. As soon as I connect them, the display duplicates to the second monitor. But I would like to extend the displays, and the Display Settings acts like there is only one monitor. It identifies both as screen 1, and if I hit Detect it says that it cannot detect another display. So, if I tell it to extend displays, there is of course no change.

However, the second monitor shows up as a device under Devices and Printers, listed correctly. There are two display drivers--I have updated both and restarted. No change. How do I convince it that there is another monitor? Thanks very much.