Only SOME local files will transfer to my iPhone.

[alexrowley1]

Plan

Free/Premium

Country

United Kingdom

Device

iPhone XR

MacBook Pro 13-inch

Operating System

iOS 13.5.1

macOS Catalina 10.15.4

My Question or Issue

I've been trying to transfer local files from my MacBook to my iPhone. All my local files play fine on my Mac. When I try to transfer them to my iPhone, only some of the songs transfer. The rest are left greyed out.

They are all MP3s.

They are all songs that can't be found on Spotify already. 

Both my Mac and iPhone are on the same network and I have also tried using a personal hotspot. 

It seems to be completely random which songs transfer and which don't. 

Pls help me