Thumbnails turn black

I'm new to Pr Pro, having just used Pr Elements for several years.

I'm in the lower left panel (Proj Panel?) where I have imported my files and selected icon display. 

When I move my cursor over one of these clips it immediately turns black.  There is a thin blue line at the bottom with a dark section that follows my scrubbing cursor, but the thumbnail remains black.

If I double-click on a thumbnail it opens, black, in the Source Monitor.  If I click on the black image in the Source Monitor the image appears and I am able to scrub through it. 

And this is where it gets even more confusing.

If I return to the Proj monitor and hover over any icon it turns black.  If I double click any icon, not currently in the Source monitor, the image in the Source Monitor remains unchanged.

But this is not consistent.  Occasionally double-clicking an icon loads that file into the Source monitor, replacing the one that is there,  but I haven't been able to work out a consistent response, like when does it load a file and when does it not.

I've watched video tutorials and this doesn't happen to them.