I've been playing around with my Rasplex for days and I came here to find out how to get all my cover art to display. It displays everywhere else on all my other devices. I've been force caching it for a while, nothing seems to pull that art.
I had the same problem yesterday after installing Rasplex 0.2.1 with a dozen of movies not showing artwork. They were consecutive in the middle of my movies library. My workaround to solve this issue was going to Plex media manager and clicking on the update option. Then in Rasplex I went back to the main menu and went to the movies the artwork was there. There was a couple of movies that this trick did not work, some of them I changed the artwork (you can change it back later) in media manager and that result enough. Some of them, I had to make the artwork change and reset Rasplex. I guess what I made with all this workaround is to force a cache refresh. This behavior did not repeat since I did the workaround.