Responsive behaviour fault

I like this plugin, only one I have found that does what I need.
Unfortunately the responsive behaviour doesn’t seem to work properly: I am trying to show just one slide at a time, 250 x 250px. This looks fine at high resolution, but below about 1220px wide it jumps to showing 4 smaller thumbnails next to each other. Below 1000px it jumps to three, below 800px it jumps to two, then back to one below 500px. I don’t know if this is the expected behaviour but I need to display just one thumbnail consistently at all screen ratios.

Using Twenty Thirteen theme.

Thanks!

https://wordpress.org/plugins/wen-logo-slider/