Album Art not showing on USB drive music library

[nipSnertz_]

I have a USB hard drive connected to my Wiim Ultra. After scanning the drive files, the albums show in the Home app and can be played. However, about 10 percent of the albums do not show the Album Art. All files are normal MP3 files and I checked the problem files in Mp3Tag and the correct album art seems to be attached to the files. Why would mp3 files with album art not show the art in Wiim ? I did a second full scan in the Wiim home app but it did not help.