If you have existing reservations, you should also check them individually to make sure that it is there. I don't believe updating it in your profile will necessarily add it to existing reservations (or correct them if it is wrong).
How do I know if it is attached to my existing reservation? When I open up "secure flight data and passenger information" it has a check and says "secure flight information on file". It has this for all three of us and only two of us have KTN's (one is a minor). None of the fields are filled in (nothing under gender, DOB, KTN, etc.). If there a way to see if my KTN is on this reservation? My KTN is listed in my profile. Thanks.