I'm faced with the same issue : using ATA 3.6.1, I cannot possibly have thumbnails on my multi posts page (home).
I've set excerpt lenght at 55 (default settings)
I'm clicking "use featured image".
I'm using now a plugin named "Thumbnail For Excerpts" BUT due to its lack of options, I cannot possibly display a full post (sticky) and excerpts.
Any ideas pls ?