Does anyone have any ideas? It makes no sense. I've figured out how to make it simpler to understand. If you look at the following URL, you can see where the tracks stop outputting. At a track 'start' of 21908, the song "I Gotta Feeling" (and every song before it) works fine, and outputs track information with Rdio buckets. However every song after it (with a song ID greater than 30963, no track information is displayed. Which means we can't query any buckets.
http://developer.echonest.com/api/v4/tasteprofile/read?api_key=_API_KEY_&format=xml&id=CAKKFGP146A91ACA17&results=20&start=21908&bucket=id:rdio-AU&bucket=tracks
Thanks guys.