Tracks not playing offline + incomplete downloads

[PTM333]

Hoping this is a well trodden path... 

Tracks repeatedly downloading despite previously showing as downloaded and available offline, and then automatically un-downloading for some reason. Tracks showing as available offline do not play offline, and Spotify telling me to get internet connection.  Inconsistent playback on some tracks on an album - some playback, others don't, and regularly have incomplete tracks stopping mid song.

I have followed all the uninstall / delete cache/data suggestions several times, but no improvement. Mobile device is an S4 mini running Android. 

Any suggestions would be very much appreciated. Thanks!