Why isn't iTunes letting me add music to my iPhone 5?

I added a bunch of songs that were on my mom's computer earlier today and they all transferred fine, but i still had more songs on MY computer so i went to transfer them but they're not letting me. I connected my iphone and registered it to the computer just like i did on my mom's computer but every time i dragged the song from the library to the phone it wouldn't transfer. It just didn't do anything when i dragged it.

In iTunes Menu
File
Home Sharing
Turn on Home Sharing

Also… Tick the button for manage music library manually. This will allow you to add only the checked songs.

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