Can't get much more specific than this. I hope this is helpful.

When you hover your cursor over a tile up against cover, these little shields pop up to show you whether there is cover, and whether that cover is half or full. When I started playing the game, this worked. When I upgraded my video driver (to a release just last week) the little shields stopped appearing; it made play extremely challenging. I rolled the video driver back to the previous version, and the little shields now appear again.

I'm using nVidia 560 GeForces in SLI (though I disabled SLI post-upgrade and the little shields still didn't show...I needed to rollback).

The version that the little shields work with: 306.23

The version that the little shields vanished with: 306.97

If you need more information, let me know.