This happened to me 3 times already. I HATE google music. The UI sucks and the cloud is really buggy. I have 12 GB of songs that are contained in 12 playlists. I synced all my songs from windows media player to google music. For the first day or two it works ok. On my desktop browser, when I open google music, I see all my music and all my playlist with all the songs. I also like to "pin" my music ( pin meaning save it to my phone because I can't afford to stream it since I have the 4gb plan). After waiting about 16 hours for the music to download everything is working perfectly for a day.
A day, and that is it. All my music that was downloaded on my phone via Google music is erased. Also, the playlists on the google music cloud itself is messed up. When I check on my desktop and open google music, I see that my playlist no longer have all the songs; for example, a folder that is supposed to have 200 songs now only has 180 songs.
Unfortunately there is no real alternative to any other wireless sync. Cheetah sync sucks and Doubletwist sucks more. Also because of the crappy MTP format of the nexus, i cant sync it either when I plug the nexus to my computer via usb. And manually putting each new song I get into the Nexus is a B*** because I have to find the new song in a folder with 400+ songs.
I must say, in terms of a media player, the IPhone kills the nexus, or any android device. There are no great alternative apps for google music.
I love everything else about my nexus. Its just that a music player was one of the important things I wanted on the phone and the nexus sucks in that department.