ITunes won't sync a huge portion of my music files to my iPhone

Question

Syncing my iPhone5s leaves enormous and seemingly random (though always the same) swaths of mp3s grayed-out ("dotted circle") or simply not copied from iTunes to mobile device (iPhone). The most warning I ever got was, I used to get a pop-up on the iPhone screen reading, "iTunes Sync -- [x] items could not be synced. See iTunes for more information." This number varied: [x] might be 24, 49, or 1,260, but iTunes itself gave me no warnings, alerts, or info of any kind.
I have restored from backup, I have restored to factory settings, I have manually deleted all music and re-synced, I have selected and unselected, checked and unchecked, manually copied and let iTunes do it, I have turned on and off iCloud and tried all of these steps again. I have turned on and off the setting to convert higher bitrate files. I have reset warnings over and over and over and over between each of these steps, hoping for SOME kind of "more information."
Somewhere along the way it stopped even telling me on the iPhone screen that files "could not be synced." Now it just goes through the motions, sits for a while thinking on Step 4 of 4 (ostensibly copying the files), and then returns control of the application and the device to me, as if it has completed the task. Nothing has changed and 1000s of files are still not being copied over. Files that WERE there on previous versions, prior to me deleting all the files in an attempt to get on just a small number of new ones.
I have tried every single base-level piece of advice I've found online or in any Apple Discussion post.
The files that won't copy don't have anything in common I can find. Furthermore, what iTunes says is on the phone and what the phone tells me it has does not match — in iTunes, if I select the phone and navigate to the playlists, for example: I am told that all tracks from the Tintin Soundtrack ("Adventures of Tintin" makes it alphabetically first and so it stands out as an example) EXCEPT tracks 6, 8, 9, and 10 are available. 6, 8, 9, and 10 are gray-circled out. However on the iPhone I only see track 1 of 18. No gray circles. Just 1 track. These tracks are all the same bitrate, same ID3 metadata, same file location. They all play fine in iTunes and I cannot see what would make one file copy several files *seem* to have copied, and some files not copy at all.
I am past angry. I feel defeated by the computer and on the verge of tears. What the ****, guys? What the f***, Apple?
I have an iPhone 5s.
iOS updated, v8.0.2 (12A405).
iTunes updated, v11.4 (18).
I am on OSX v10.6.8, 2.16GHz Intel Core 2 Duo processor, 2 GB 667 MHz DDR2 SDRAM memory.
I have my iTunes library spread across two external HDs, but this has never given me trouble in iTunes (well, it's incredibly slow ever since v11 came out, but it finds and plays files 100% of the time, in iTunes), and again, there is no pattern I can find between the files that do and do not copy. Certainly not location or drive or anything (alas).
Please help...?

Answer 1

I had this problem.  Then I changed some of the music in my iTunes (manually going through to make an album "part of a compilation" just because another Artist happens to contribute to a song, so iTunes assumes its a different album?).  I then tried to sync but only some of my music came across, got the circles too.  I did a restore of my new iPhone 6 Plus and setting it up as a new phone and synced again, but then noticed some other albums needed updating on the library so did that, tried to sync and it won't.  Same songs not coming across as before.  I am in the process of deleting the library from my phone and will try again.  Otherwise, I have to do another restore and try again.  Will have I have to restore and start from scratch every time I manually update my library?  iTunes needs sorting out!!!!