iTunes not syncing all the tracks

[jackoverfull]

I’m still on Mojave, so I still use iTunes, I’ve recently noticed that it’s no longer syncing all the songs on my iPhone SE with iOS 14.

I have a vast library, so I can’t sync _all_ songs, instead I sync only selected playlists. I’ve noticed that one playlist doesn’t appear on the iPhone, even if it shows up in the selected ones in iTunes, and the songs are not transferred over. If I duplicate the playlist BOTH appear, but if I delete the copy both gets deleted.

Also, I just exported twice the same Logic project (wrap different variations of the same song), put the two files in the same playlist and one gets copied, the other doesn’t.

On my other iPhone synced the same way, an old 3GS with iOS 6, all works fine.

 

[jackoverfull]

I believe I found the solution: for some mysterious reason some tracks appear as missing in my iTunes library, while in fact they are right where they should. Trying to play them TWICE fixes the problem and then they are transferred to the iPhone correctly. Why the were being transferred to the other iPhone or how did this happen is beyond me.

After a while of trying to solve the issue manually it apparently solved it automatically and now it's syncing over 1000 tracks! Just 10 tracks or more were still missing and it was easy enough to locate them manually.