Since upgrading to iOS 8, many users have reported issues when it comes to syncing music with iTunes. For some reason, all their music is gone and no matter how many times they try and sync, no music ever gets transferred. Here's what worked for us!
Before continuing, you will want to make sure that you don't have iTunes Match enabled in Settings > Music. If you do, keep in mind that it will always override whatever you have hard synced from iTunes. Actually, iTunes won't even let you attempt to sync music if iTunes Match is turned on. If that's the case, simply disable iTunes Match and try syncing again with iTunes like normal.
If you don't have iTunes Match or already know it's disabled, continue on.
How to fix music syncing issues with iTunes and iOS 8
Plug in your iPhone or iPad running iOS 8 or higher and launch iTunes.
Select your iPhone from the dropdown menu.
Select Music in the side navigation panel.
Uncheck the box for Sync Music and then click Apply or Sync.
Once the sync is complete, recheck the box for Sync Music and hit Apply or Sync again.
Unplug your iPhone or iPad and see if your music is back.
For most folks, this fix seems to be doing the trick. It worked for me on my iPhone 6 Plus that didn't seem to want to sync any of my music from iTunes. So give it a try and let me know whether or not it works for you! And if you've found another way to fix iTunes syncing issues in iOS 8, be sure to let me know that too!