I updated to iOS 7.1.2 on my iPhone 5 and less than half of my music is syncing to my phone. Any ideas? I already start to hate this update for more than one reason and this is adding icing to the cake.
Not confined to iOS 7, syncing has been a known bug ("ghost songs") in every iOS update. The following paragraphs will tell you everything you need to know about this problem in 2 parts.
Download and install the latest iTunes. Disconnect your iPhone (or iPad/iPod), re-launch your iTunes and connect your iPhone in several minutes (or have a reboot). During the process, you can also go to "iTunes Store", choose to deauthorize and authorize your computer. Be patient and you are likely to let it work.
With your iPhone plugged, go to your iPhone tab, deselect "Selected playlist, artist, albums and genres." Then switch down to genres, select songs and hit "Sync".
When connecting your iPhone (or iPad/iPod) to iTunes, directly drag a song to your iPhone. Then hit "Sync". Though this method works for some users, I can't imagine syncing hundreds of songs to iPhone one by one. No one would ever bear that!
Even without iTunes, you can still sync music to iOS 7 or iOS 8/9 devices, as iPhone 5 Transfer could help you easily transfer all songs to iOS devices. This iOS transfer tool enables you to transfer not only music but also video, playlists, photos, contacts, SMS, etc. to your iPhone, iPad and iPod. Before you get started, download iAny Transfer to your computer. We'll take iPhone 4 for example to show you how this is possible.
Additionally, iPhone 5 Transfer allows you to create new playlists for your iPhone 5s, iPhone 5c, iPad Air and other devices. Just head to the Palylists tab and click the Add Playlist button to build a playlist and manage your music files.