Trouble syncing new music to iPhone in Windows

I added some music to my iTunes library, but when I attempted to sync it to my iPhone, the new music does not sync.

When I open my iPhone library within iTunes, the new music appears in a gray font with a circle showing in the far left column. The new music does not appear on the iPhone itself.

All of the software is uptodate -- Windows, iTunes and iOS. The PC recognizes the iPhone. When syncing did not work, I triEd to manually add the music, but that did not work.

I have spent about 90 minutes on the phone with Apple trying to resolve this problem, but we had little success. We checked the software was uptodate, we added the iPhone as a new device, etc., but no luck.

Does anyone know what the problem could be?