Downloaded songs

[Michaelnauss]

I have many songs and playlists downloaded for offline use on my android phone . I can use them for a bit but it always seems like they become not downloaded after a few days and then I have to re download them again. This is extremely time consuming and plus I end up using data by mistake because I play the songs offline not realizing that they are not still downloaded.  Please tell me how to fix this. It should not be happening.