This could be do to several reasons, here are some things I would change.
Your URL Structure:
http://www.everynote.com/flute.music
Should be
http://www.everynote.com/flute-music/
or
http://www.everynote.com/flute_music/
But that has really nothing to do with the rank, unfortuently to go from a PR4 to a PR5 is a ton of hard work prolly close to 500 pr4 backlinks or 2500 pr2-3 backlinks.
They take a long time, but before you know it you will be their.