svantana,
You are using the Spotify URI correctly. We do not yet have audio analysis for all the Spotify tracks, which is why the audio_summary is sometimes empty. You are doing the next best thing to use the song analysis.
As to some tracks not being found, this appears to be a bug and we are looking into it. In the "Latch" case, that track can be found (from a different album) at:
http://developer.echonest.com/api/v4/track/profile?api_key=API_KEY&format=json&id=spotify-US:track:1BltsyC5W3SAABdxyrDXwi&bucket=audio_summary
Thanks for pointing it out!
David