Gallery Type Post Missing Thumbnail

I’m using 3.9.3 with theme Twenty Eleven version 1.7

If I make a Gallery type post and the only pictures I include have been previously used in another post or page of the site, then a thumbnail will not be shown with the post teaser.

My work around has just been to re-upload a second instance of the picture that I want the thumbnail to be and use that in the post instead. Figured this might help someone out and might be good for someone to know about this behavior.