Downloaded songs stopping after 1sec when offline

[JLSLM]

Hi

I am new to Spotify premium. I only use it on my iPhone 8.

Some songs are stopping after 1sec when playing offline even though I downloaded them while on wifi so could play them  offline. It seems to be only the songs I imported that were already on my phone (previously played via the Apple Music app) that have the problem. These are songs/albums that were available on Spotify but given the import option was there I thought it would be quicker than searching for each album in sportify and downloading. Maybe in hindsight I should have done that. 

Also, I imported songs not on Spotify to my library. Why can’t I search them up under “artist” etc they seem to show up when songs shuffled but given the prob above don’t play past 1sec anyway. 

Any suggestions? I am hoping I am missing something simple. 

Thanks