Some songs sync/download offline, but other's don't. WHY!?!

[christophernyck]

I have a bunch of songs purchased and stored on a hard drive that I purchased on iTunes over the years. Some of these albums are not available on Spotify, but I point Spotify to the drive and it finds and syncs MOST of them. However, SOME of the files stay greyed out, even though the corresponding .mp3 files are sitting on the drive.

Making things worse, when I sync/download these same files to my iPhone, most of the ones that synched properly on the desktop sync properly on the iPhone -- but a whole bunch of them don't.

I have tried everything to try and resolve this. Clean install, deletion of data, clearing of cache, rebooting of devices, switching wifi networks -- done of it works. 

What am I missing here?

My info:

Plan: Premium

Desktop: iMac

Mobile Device: iPhone XS Max