Why can't itunes sync my ringtones to my iPhone?

I have an iPhone 3G S and iTunes version 9.0 or higher. I tried to create ringtones for my iPhone like I did 3 days ago but this time it won't work. The first time I tried it, it didn't work but after an hour I somehow got it to sync. I'm pretty sure I did everything right. I made sure to change it so the song was no longer than 40 seconds and I converted it to AAC version. Then I changed the file type from .m4a to .m4r for ringtones. When I played it, iTunes recognized it as a ringtone because it played under that tab but once I tried to sync my iPhone to it, it wouldn't go it. I thought it might be something wrong so I tried it with the ringtones I created before and it worked fine, it's just that new one I'm trying to make. It's not only that one song but all songs! Can anyone help me? It's really aggravating and it keeps bothering me.