The android music 'system' is something that needs an overhaul, seriously.
Try going in settings / manage apps, go in media storage & clear the cache. Maybe even clear the app data (start afresh). Maybe you could also unmount / re-mount the SD card, so that the media library will rebuild itself. Be patient while it rebuilds, on my phone it takes about 15 - 20 mins (about 14gb music).
Doing the above usually sorts things out. Alas, I have music in mp3 AND AAC & for some reason Android music doesn't like AAC tags (mp3s are fine). Sometimes it will make a mess of compilations (mix CDs) & duplicate them, so I have to do the steps above.
Even cheapo mp3 players behave better than this, it's the one thing that let's Android down.