Sometimes, if you have an uncommon version of a song, the app might not recognize it and can't show the lyrics. If this happens, there are two things you can do: either write the lyrics yourself from the application, or tap the browser button to search for the lyrics somewhere else.
Lyrics Grabber is an excellent app for getting music lyrics. It barely takes up 500 kilobytes and works perfectly. The interface is a little rough, dark, and simple, but it gets the job done.