Why is my Album Cover Art gone from my MP3's?

I have tons of album folders, and I have an Album Cover Art picture for every single/song in the folder for each containing Album. About a month ago they all went back to saying "MP3" or "WMA" for each song/single, etc. I still have the album cover art, and it shows up in Windows Media Player when I play it or paste it on the singles through WMP (Always shows up in the player), but now all the songs in the Album folders show no Album Cover Art. I tried to redo some and it worked if I changed it from A WMA to an MP3, and now none of it works (Rather it's MP3 128, 192, 256, or 320, and even any type of WMA). I need to know what is causing that in simple terms so I can hopefully resolve it. I am very picky about my MP3's and love the art work showing up on each single/song. Thanks for any help in advance.