You said that everytime you log yourself in on Spotify, all downloaded tracks are missing. So a little question for my better understanding of it: Do you log yourself out every time? So don't you just close the app?
Because if log yourself out every time it could occur that downloaded tracks are missing!
If that's is not the problem, I suggest you to do a fresh reinstallation of the app and start fresh all over!
I hope I could help you a bit out with that and if there are any questions left just ask me!