Playing songs online and offline

[kiireii]

1. Plan

Premium

Country

Device

iPhone SE

Operating System

iOS 10

My Question or Issue

I’ve had Spotify for about 4 months now and I still can’t solve this issue. I download songs and you can see that the whole playlist is downloaded (the download sign is green) but when I play almost any song, it doesn’t play it. Sometimes it does but only for about six seconds. I even have really fast wifi. When I go offline I can’t play literally any song. Please tell me how to fix this, Spotify. Thank you.