Local files, synched from desktop - disappear

[spitspotpremium]

(this goes for both android and iOS devices..)

I was wondering.. once in a while my local files that I've distributed to my different devices (from desktop spotify); they disappear and need to be re-synched, quite often actually... Why? Is it everytime a new update or.......? Or is it every 14th day or something?!

Why can't I KEEP my local files (that I own) - and synched from my desktop to my spotify player on other devices?!

It's not very reliable when I'm travelling and I suddenly don't have them on my device (other than desktop) !?.. 

hope it makes sense...

cheers

(premium-user)