Hello Simply b,
Thank you for using Apple Support Communities!
From what you describe I have a couple of recommendations for you. First, double check to make sure the Sync Music checkbox is checked in the Music tab of your device when it is selected in iTunes.
- Open iTunes.
- Connect the iOS device to your computer using the included USB cable and click the device button in the upper right corner. (If you are viewing the iTunes Store, click the Library button in the upper right corner. The device button will then be visible.) Some tabs may not appear if you do not have corresponding content in your library. For example if you do not have any podcasts in your library, the corresponding Podcast tab will not appear.
- You can configure the different content types in each tab. For example, Movies under the Movies tab.
- To sync, click Apply, in the lower-right corner of the screen.
From: iOS: How to transfer or sync content to your computer
If it IS checked, then uncheck it, and click apply in the lower right corner. Then restart both your computer, and the iPhone, recheck the checkbox to sync your music and try again.