There are a few reasons why you are unable to get new songs:
1) You stop receiving Case Coins to claim Song Cases when all the collection songs are unlocked. Once new songs are added to the game you will start receiving them again!
2) Journey Cases become unclaimable when all the collection songs are unlocked. Once new songs are added to the game you will be able to open them!
3) You have received all the songs available from your unlocked ranks (you will no longer have an "available in boxes" section in your song collection). Win more stars to unlock a new pool of songs in the next Rank.
If you've collected all the songs, keep an eye out on the in-game newsfeed for new song announcements. Let us know if you have any suggestions for songs you'd like us to add to the game!