I may have found a workaround for missing music. I should note that this helped me when adding new music from iTunes using the windows phone desktop app. After all your new music has been added via windows phone desktop app, unplug your device, then delete ALL playlists, then plug back in to your pc and re-sync. Depending on how big your playlists were, it will take about 5 minutes to refresh and discover all your new music in Groove. You must have background enabled for groove, keep the app open, and make sure your phone is plugged in to an outlet or at least be above 20% (this is for battery saver). It seems there is a problem that causes the groove app to hang if your new music is part of a playlist. It worked for me at least with iTunes. Hope they get this sorted soon. It's kind of a way to force a refresh of new content. Good Luck!