Phone model: Sony Xperia Z3, Android Lollipop ver. 5.0.2

I have a premium account and downloading songs have been convenient for me especially when I'm on holiday vacation or taking the subway. But lately, all of my saved songs would spontaneous "undownload" themselves from my library. The songs would still appear in my library but show as if I didnt download them at all. This is problem have been tormenting me for the past couple of months and it's annoying. I'd even uninstall and reinstall the app MANY times and it still continues to happen. I'm tired of having to re-downloads every song on my library and I will cancel my subscription if it continues. Seriously Spotify, GET IT TOGETHER!!!!