Windows 7: Media Player not consisitantly displaying library of music

I am running Media Player 12 on Windows 7, 64 bit. I have a lot of music in my music library, which is also selected as a location for media player to "look at" under "manage libraries". Most of the time when I open Media player, only the sample music originally installed on my computer shows up. Sometimes, if I wait a long while, media player says it is updating my library, then eventually my music appears. Sometimes it doesn't update or appear at all. Note: if I search for a song, album, etc... it will find it and begin playing my music and adding it to the library AGAIN. Or if I just open it from my windows music folder, it will do the same. Why can't I just open media player and see all my music?