Some of my downloaded songs will not play in offline mode and I do not have unlimited data.

I have downloaded around 1500 songs, they are all saved and have the green tick next to them to show they are downloaded.

Most of the songs play fine in offline mode but around one third will not play at all. 

When I click on the song it will not play until I put the phone in online mode. 

I am using an Android phone running version 6.0 and Spotify version 8.4.45.633 armV7.

I have around 50Gb free on my memory card and the phone is the only offline synced device.

I have tried uninstaling the app and downloading all the songs again multiple times with no success.

Any advice? as I can't find an answer anywhere and I'm tempted to move to Google play.

Thanks for any help you can offer.