sometimes the download will not start or not complete if there is insufficient space for spotify to save offline (which ordinarily is 500MB-1GB of free space).
sometimes, if spotify is outdated, an update is completed, or indeed if the OS is outdated, downloading issues may arise. so make sure your android and spotify is updated.
Often something occurs with the offline files on your devices. The cache may become corrupted, or mis-register what songs are available to download, and disconnect from the app.
If you follow this link you can reset your offline devices. Then you'll need to perform a quick reinstall of Spotify with these steps (make sure you restart your device after un-installing).