Screensaver quit working?

I have a box running Kali with the MATE desktop. I just switched to a new wiiiiiide monitor, and my screensaver no longer kicks in, ever.

I have tried refreshing its settings, etc. If I invoke it manually with Super+L it starts. But it never powers off the monitor.

What could cause this? What else might I try to at least isolate the cause?

#linuxquestions