Metro Scrobbler works well for me, but make sure you do the right steps when trying to scrobble.
just listen to music in the xbox player on the phone (either music you've synced from your computer, or music downloaded off the xbox music store) and then open the app after listening. it will scrobble anything that you've listened to since your last scrobble in the app.
make sure to swipe over to go to "options" in the app, then the "played" tile. then choose the "sync" option on the bottom popup menu. there might be an alternate way to scrobble, but this has worked for me.
it would be cool if there was a way to scrobble music listened to in other apps such as "Amazing Music"