Redownloading music in playlists on Apple Music

I am curious why I seem to have to keep downloading the songs in my playlists to my phone from Apple music. I use an iPhone 6+ and have the latest software update. This has happened a few times now over the past couple of months. It seems to erase the songs on my phone and I have to download them all over again. Does anyone know how to make sure that they are kept and I don't have to down load again? Thanks for the help!!