Hi! Welcome to the Spotify Community!
I hope I can help you solve this frustrating problem.
- Could you try to make a small playlist availible offline (if this doesn't work either, let us know in a reply)
- Depending on how new your iPhone is it might be possible that your device is low on working memory or memory in general, please try to close all background applications by double tapping the home button and closing all apps by sliding them away.
- Last option, look in your genereal settings via Settings > General > Usage > Storage and make sure that you have at least 10% free space (so 1.6GB for the 16 GB iPhone)
I hope this helps, let us know!