For many years I have been unable to fully sync my iPhone to my iTunes music collection on my MacBook, as there were a number of songs that, for unknown reasons iTunes would refuse to sync.
The main symptom was, in the list of music installed on the phone displayed in iTunes (on the Mac) a bunch of them just showed up "grayed out" and no amount of right-clicking (or googling the interwebs, for that matter) would provide any hint at what was the root cause:
How can one fix the issue and get the songs finally transferred to the iPhone?