Why do I have to keep re-downloading my music?

[Dalby]

Questions in the title.  

I tend to download full albums to my mobile for use in the car.  Mobile app is upto date and it is happening on the htc one m8 but to be honest it always used to happen on the S4 aswell.  I am a paying user.  I have no local music its all from spotify.

The odd thing is its never full albums that get deleted just a track or two.