Can’t find a fix for 'This song is not available in your region' on Apple Music...

I use Apple Music purely to play MP3s on my iPhone, in the

UK, which are transferred from my PC via iTunes. I constantly find certain

tracks aren’t transferred, or won’t work, and get the alert - 'This song is not

available in your region'. However it’s often random and the next time I

connect my phone some of the tracks will transfer and work, I’ve even had voice

recordings that I’ve made myself which generate the same alert - 'This song is

not available in your region'. My Apple ID is set to the correct country.

So far, I have tried all these fixes –

1 ATTEMPTED FIX

Go to: Settings > Account > Media & Purchases > View Account >

Country/Region and in the "Media & Purchases" menu, sign out and

then sign in again.

RESULTS

It worked on a few songs and then they stopped working again.

2 ATTEMPTED FIX

Update iPhone software and iTunes.

RESULTS

No noticeable improvements.

3 ATTEMPTED FIX

Reset warnings in Apple Music or iTunes.

RESULTS

No noticeable improvements.

4 ATTEMPTED FIX

Sign out of your Apple ID account and then sign back in.

RESULTS

No noticeable improvements.

5 ATTEMPTED FIX

Change Apple ID country or region.

RESULTS

I’m in the UK and this made no difference.

6 ATTEMPTED FIX

Slightly alter the title of a mp3, e.g. add ‘new version’ to it.

RESULTS

This worked for a few weeks and then stopped again.

Has anybody got a fix for this problem that works?!