Local files sync to iPhone no longer working for me

[hurricane_jane]

Plan: Premium (Family) 

Country: UK

Device

iPhone 7 and MacBook Pro running High Sierra

My Question or Issue

Hi! So I have always used Spotify's fab 'Local Files' feature to add songs from old folk music CDs from the country where my parents grew up (which is unsurprisingly not featured on Spotify!) but it is suddenly no longer working. 

Around a month ago I deleted my existing playlists featuring local files because I had run out of space (duh, iPhone life), but now I have cleared a fair bit of space and I want them back, but I can get nowhere near adding them. I have followed all the steps from various Spotify community threads and articles but nothing helps - the songs don't even show up and I don't know why. Seeing as these are files from CDs that were bought about 20 years ago I don't see how the 'illegal files are not permitted', which appears to be the only explanation, could apply...  I should add that my parents are also still happily listening to these files on their Spotify iPhone apps and having no issues. 

Is it perhaps the case that my phone just still does not have enough space (despite having way over the recommended 1GB available)? Pls help before I move to Apple Music lol