I have owned my galaxy nexus for over three weeks now and this has happened to me at least 5-6 times: suddenly, the 20-something gigs of music that I download over wifi is no longer available offline, and the remaining storage on the phone increases by those 20-something gigs.
Things I have tried to fix the issue:
- I once did a "Force Close" on the app followed by a "Clear data", re-started the app, re-linked my google account to the app, then re-marked the music I like to be made available offline. The music gets re-downloaded. Inevitably, sooner or later, the music that I make available offline becomes no longer available.
- I have uninstalled the 12/15 update to Google Music to see if the stock Google Music app does not have this issue. It does have the issue.
The next thing I will try is to not try to charge my phone via USB connection for the next few weeks to see if has to do with the phone attempting to serve as a storage device.
The application would be great if not for this issue (and the fact that I cannot double-click the button on my earbud remotes to fast-forward a song quite often).