Hello there Eden,
Thank you for using Apple Support Communities.
It sounds like your iPhone seems to be sycning in iTunes, but the new music is not transferring over.
I would recommend a few things.
First, restart both the computer and the iPod.
To restart iPod touch, first turn iPod touch off by pressing and holding the Sleep/Wake button until a red slider appears. Slide your finger across the slider and iPod touch will turn off after a few moments.
From: iPod Touch Troubleshooting Assistant
Other iPod models:
How to reset iPod
I would next try resyncing your Music to the iPod. You can resync content to the iPhone by unchecking "Sync Music" and then clicking "Apply" and then checking "Sync Music" and then clicking "Apply" again:
I recommend reviewing the full article here: