Why can’t the app find my music?

If the Play-Fi app is unable to find tracks that you've downloaded to your phone, tablet or a networked media server, check the list of supported file formats first.  If your music is in an unsupported format, it will simply not appear in the library.

If you've confirmed that the missing files should be supported, but you still can't see them:
Android 

For iOS

For Media Servers