missing artwork tab

I've been recently converting and tagging a number of my movies to mp4 however the "artwork" tab is missing for some of the movies. I right click and get info but the tab for artwork is missing. I checked the permission and tags and I don't see anything different. Any ideas what's going on? It's only happening on a few movie files and my workflow for every movie is the same (just using mkvtomp4)

Must be some type of bug since I found a workaround. I grabbed two movie files and chose "Get Info" Then I dragged a picture into the artwork placeholder thereby changing the artwork for both. Then I reselected my original movie and the artwork tab was now there...replaced image with correct image.