If you’re using your Windows Media Player (WMP) as a Windows Media “server” to share your music library, yet don’t use WMP directly to manage your media (I use the much more rich MediaMonkey) then you may want to force a library rescan when you add new tracks to your file-store(s).
This isn’t very hard, but it isn’t entirely intuitive either. However it’s simple enough. Select the WMP window, hit the “ALT” key and a menu will pop up. Select “Tools / Apply media information changes”, eg:
Should do the trick.