I want to amend my earlier comment. I converted all of my music to mp3 files, and I'm having the exact same problem. About half my collection won't sync to the GNex. So it doesn't matter which format the files are in. If I try to specifically sync one of the Albums that didn't sync, WMP says that the files are already on the device. They're not. Confirmed with file manager. I'm confinced that GMusic does something to spoof the operating system into thinking that music uploaded to GMusic is already on the device, so that it shouldn't be copied. I've tried WMP, Winamp and DoubleTwist. They do the same thing. If I manually copy the files to the GNex, they don't show up in GMusic. If I make online music available, and check the option to make such an album available offline, it downloads the music, but then doesn't show up as soon as I tell the device to only show local music. This is driving me off the edge!