Siri doesn't really work with the Music app (officially, it's not supported). It's hit and miss, sometime it will find a selection if that title happens to be currently displayed on the TV screen (but not if the same title is just off screen), sometimes it will ignore the song that is currently shown and display a match to a movie on iTunes or Netflix, sometimes it will find a title and then select it but not play it, and sometimes it will say it can't find anything or respond with an attempt at a humorous quip.
Basically, it's a complete mess and only "works" sometimes because there are multiple search paths that Siri will attempt if it can't find a match to something in the services that it does support (like in iTunes Movies and TV, HBO, Netflix, etc.).
In fact, the entire Music app is a mess, buggy, and lacking key features that it obviously needs (like a way to quickly skip to a track that appears WAY down a long list of songs, no way to shuffle play in Genres or Recently Added, no way to select the artist for the currently playing song, etc.).
