This troubleshooting article explains how to fix the issue of iTunes not syncing songs back to your iPhone or iPod. In most cases, the problem is caused by misconfigured iTunes sync settings for your iPhone or iPod. The guide works with all iPhone models including iPhone 6s and all versions of iTunes, including iTunes 12.
iTunes not syncing songs to iPhone, iPod, or iPad
Run iTunes and connect your iPhone, iPod, or iPad to your PC. Once the iDevice appears in iTunes, click on it
Next, click “Music” on the left
If iTunes says “iCloud Music Library is On”, turn it off. To do so, from the iPhone screen, navigate to Settings > Music and place the “iCloud Music Library” switch to the Off position, next reconnect the iPhone with iTunes
Back to iTunes, make sure that the box next to “Sync Music” is checked. Also ensure that the option “Entire music library” is selected and click on the “Apply” button
iTunes should now transfer all the music present in your library to your iPhone, iPod, or iPad.