Play Music (KitKat): update library not working
One of the advantages of a physical SD card: unmounting and re-mounting it triggers the media scanner. But that's unlikely to be an option for you. 1

Depending on your Android version, there are different apps available on Google Play to help you out. The "version difference": starting with Kitkat, the old mechanism of triggering the media scanner does no longer work, "normal apps" are denied the permission to call it. But there's at least one new app available where the dev found a "loop-hole" to give back the functionality.

Some examples for pre-Kitkat scanners can be found in the link of "footnote 1", SD Scanner is the one promising to work with Kitkat.



1 see my answer here for a more detailed explanation of the background