I use both the Windows version and the app on my phone. I have numerous playlists and although all of the songs show up in the desktop version in these playlists, many of the songs show up as unavailable in the app. If I switch devices on my phone to control my laptop, the phone can launch one of the 'disabled' songs to play on the laptop so it appears to know what it is.
Also, I frequently have the problem that other users report of search not working becuase I'm offline (even though I'm not, all other apps and services are working on my phone at that time).
This is a recurring issue so has nothing to do with restarting. I have the latest app (just uninstalled and re-installed today) and have the latest on my Windows laptop, too.
Enjoy having the music but this has got to be one of the buggiest apps I've ever used.