Why has iTunes not found/downloaded all of my purchased songs?
You would need to work with Apple support to track down one specific case - but it could be that items are no longer available for re-download in your region. 500 seems like a lot of churn, but Apple doesn't guarantee you a re-download from the cloud so you might want to get your backup copies and restore them that way.


https://discussions.apple.com/thread/7101766?language=en
https://www.apple.com/legal/internet-services/itunes/us/terms.html


To contact support check your country here and then use the link at the bottom of the page: https://support.apple.com/en-us/HT204411