I can't get album artwork from itunes anymore, why?

OK, I've looked at all the posts on this across the forum and this is what worked for me - it turned out to be a format compatability problem.. I have Itunes 12 on a Windows 7 platform and have a mix of stuff I bought from Itunes (artwork no problem with download from Store) and other stuff I imported as WAV files. Some non Itune purchased files did actually respond to the "Get Artwork" command but some (even though they were in the Itunes store) did not, and neither could I drag and drop artwork saved as JPEGs on my PC. The solution for these was to click on the album in Itunes so it shows all the songs and then choose "create AAC version". This populates the song list with duplicates of the tracks already there with a new AAC version underneath the existing one - I guess you could leave both in situ although I chose not to do that, but highlighted the old tracks and deleted them to the recycle bin, leaving me with just the newly created AAC tracks. I then chose "get info" from the dropdown menu (clicking on album in Itunes) chose the artwork tab and the "add artwork" button and then double clicked on the target JPEG and lo and behold!, artwork that was previously unresponsive migrated no problem.