Resizing page reorganizes columns, but doesn't go to 1-col at smallest size

I have a 3 column grid of linked youtube video thumbnails working OK at larger screen sizes. When the browser width is reduced, it jumps to 2 columns as expected. But as downsizing continues, rather than eventually going to 1 column, it retains the 2 column arrangement and just keeps getting smaller. (Same result when checked on a small mobile phone.)

What needs to be done to get it to break to 1 column?

--Thanks--