I have had this happen to me on 2 flights recently while the other 3 family members with me had no problems. Both times I was already at the ID checker and both times I just stepped aside and was able to get a new boarding pass with precheck in 2-3 minutes. I re-added the KTN to the passenger information under the reservation and checked in again on the mobile website (not the app) to get a BP with precheck. Then refreshing the app showed precheck. Just adding the KTN under your alaska profile will not update the reservation. Somehow my KTN dropped from my profile. I've added it back to my profile and it seems to have stuck for new reservations.