Is the Art you want shown the first Artwork listed in the Properties > Artwork tab?

Is Preview set to the right tab (either Selected or Playing depending on what you're trying to achieve) and does right click on Preview > Images have multiple options (and selecting a different one works)?

I have some songs with album art that doesn't display in MM, but it does in other places (even in MM.) I've tagged them using Mp3tag, so I know they have art, and in fact if I check the Properties screen, the file clearly has art embedded in the tag, and MM even displays it there. But it doesn't display it in the art preview window.

Version is 5.0.1.2433, Windows 10.