thunderbolt display won't turn off

I'm using a Thunderbolt display, bought just before Apple discontinued them, with a late 2014 Mac Mini running OS X 10.11.5 (El Capitan).

After having it for a week or two, the display has stopped shutting off when the Mac mini is idle. In Settings, I have "Turn display off after:" set to 5 minutes, but it will sit for hours and not shut off on its own.

Any thoughts would be appreciated.