For the past three weeks, since I acquired my iPhone 6 Plus, I have been unable to sync music between my iPhone and iTunes on my Mac Pro.
I'm a weirdo in that I detest wireless services. When many called for and then rejoiced when Apple added wireless syncing to iTunes years ago I rolled my eyes and ignored it. I've always preferred ethernet connections for Internet over WiFi (for speed - which is why I went to the trouble of having my new house wired for CAT6) and wired connections when possible for devices. Anyone who has seen my office in my house will know that there are wires everwhere. Yes, it looks horrible. I know. But you know what? They always work. And they're always fast (assuming newer ports/buses/technology).
I prefer to sync my device via wire because I have a dock at my desk that I've always had my iPod or now iPhone on for more than 13 years. I prefer the speed of this and reliability of knowing whenever my iDevice touches the dock, it has synced (and performed a backup). For many years this was also the only way to do an encrypted backup, which was the only way for a backup to save all of your app login/passwords when backing up.
Despite all of this, since the first iPhone came out in 2007, I've had to give up my 80GB iPod classic for a 16GB iPhone, then 16GB iPhone 3G, then 32GB iPhone 3GS, then 32 GB iPhone 4S, then 16 GB iPhone 5 (long story - my company bought it for me), and then 32 GB iPhone 5S. Because of this I've had to selectivley sync certain playlists or artists or genres or whatever in order to have music on my phone. I wasn't willing to shell out the prices to have the 64GB and then 128GB versions of the iPhones until now. And if you suggest a streaming music service to me I will throw things at you. It goes without saying that I am not a fan.
For the past 18 months, I've worked from home full-time and since I've noticed I have almost completely stopped using my iPad around the house. My time is split between my Mac Pro and my iPhone. Because I don't think I'll be buying iPads anymore, I felt okay with paying the extra money to get the 128GB version of the iPhone 6 Plus. And because of this, for the first time since 2007 I now have enough free space on my iPhone to select the radial button that says "Sync Entire Music Library".
However, about a week after getting the new iPhone 6 Plus I attempted to listen to music for the first time and found large sections of playlists missing. The songs were listed in the playlist, with metadata present including album art. When the song attempted to play it would skip the song to the next in the playlist....which would then also skip. This would happen in succession song after song, at an incredible rate. As many as 10-40 songs would skip past rapidly before it finally found a song that was synced or the playlist would end.
After searching for information about this for approximately two weeks, I saw a tweet by Rob Griffiths which said he was experiencing a sync issue that sounded a lot like mine. After passivley monitoring Rob's timeline for about a week (he was in touch with Apple support and had been escalated to an engineer within Apple who works on iTunes) he managed to solve the issue. He wrote a post about his entire ideal with detailed instructions of how to fix this issue if you're also experiencing it. After using both the "Display Duplicates" feature within iTunes and the Dupin app he reccommended, I also solved this issue for myself. If you are also experiencing this, go read his post.