How do I add a custom text tone?

I just recently learned how to add a custom ringtone to my phone and thought I'd use the same process to make a custom text tone. Even though BOTH files show up in my ringtones section of iTunes, I can't choose one of them for a custom text tone. Anyone know how I can make a custom text tone or move one of them so that it appears in the list of tones for texts? I should mention they are both .m4r files