All of my music is no longer offline and have to re-download it

[SoundElemenets]

I opened up my phone today to find that for some reason spotify had logged me out. I managed to get on a wi-fi network and login to be shown that spotify no longer has all of my downloaded music. I understand that I can do the whole clean install of spotify to fix this but I've had this happen a few times over the past few months after clean installs and to be honest I'm sick of having to do this. The biggest problem is the SD card still has all of the songs donwloaded on it since it's using most of the SD card. Why do I need to redownload all of my music when I already have it all on my SD card but spotify just doesn't want to see it?