My self-made ringtones aren't syncing to my iPhone 4. Help?

I had a white Macbook, and I had to replace it so I got a new Macbook Pro. However, the ringtones that I'd made (from creating an ACC file of a song component and converting it to mp4) won't sync to my iPhone 4, yet they show up in the Ringtones section in my iTunes library. I have remade more ringtones on this computer, but nothing is different and I can't find any helpful troubleshooting advice anywhere online. Suggestions?