Why does my Google Play All Access downloaded music keep deleting by itself?

[AC Question]

This is the second time this has happened. I have a Google Play All Access subscription and download a bunch of albums to listen to offline. Everything works fine for a couple of days. Then this morning I shut off my phone and just turned it back on and about half of my music library says that it has to be redownloaded. Why does it keep deleting. This happened twice so far. Please help!