I have had this problem on and off for a while now. Syncing local files is one of the main reasons I pay for Spotify, but it doesn't always work. Other streaming services are starting to look more interesting....
Every support request about this problem leads to the same BS response and advice: re-installing the app and spend hours re-downloading gigs worth of your offline music again. Sometimes that fixes the local file problem, but then an app update will break them again.
Spotify, this is extremely frustrating. If you are going to advertise a feature to get paying customers, make sure it actually works. Don't keep breaking the feature with updates (have an actually data migration plan that doesn't screw over your paying customers), give users more insight into why specific songs won't sync (the generic, useless error messages are infurating), and give users more control over the cached data that seem to cause the problem (e.g. let me clear the cache of local files to trigger whatever needs to be triggered to re-download them instead of always having to re-install the whole application and re-download every single offline track).
Also, there is very little insight into why mobile Spotify clients sometimes can't connect to desktop clients. The desktop and mobile clients should be able to tell users why their mobile device is not connecting to their desktop client; a diagnostic mode (that checks for open ports, network settings, etc...) would be welcome and would lead to less infurated users like myself.