It is getting the cover art fine. However it is only grabbing the "top" cover, which sometimes isn't the desired one. Does that make sense? It would be great to be able to choose the cover from all that tmdb has to offer for a particular title. This would be in replacement to going onto the website, finding the movie, going to covers, and copy/paste in MetaX.
Something similar to the way Kodi / XBMC does it where you can choose the art from whatever is available on tmdb.