How are the thumbnails automatically generated (and why do some maps not have them?)

Discussion created by bgfield on May 14, 2013
Latest reply on Jun 10, 2015 by pml
I have a number of maps created and saved on my ArcGIS Online account.  When I save the map - in most cases - the thumbnail is automatically generated based on the extent of the features; it shows the basemap and the layers added to the map.

For some of my maps, though, there is no thumbnail at all.  I also tried changing the basemap on one service, but I didn't see the change reflected in the thumbnail.  The only idea I've come up with so far is that all of the offending maps are using large layers (>2mb in size); but, not all of the maps with large layers are experiencing this problem, so I'm not sure if that's related.

I know I can add a thumbnail image myself, but taking a screen grab of the map and shrinking it to fit the thumbnail requirements is a cumbersome process that I'd like to avoid if possible.