I just had the same thing happen for a trip tomorrow - I printed our boarding passes and my wife's had pre-check and mine did not - .... So, I go into my profile and sure enough, for no reason, my KTN disappeared. I put it back in the profile, called DL and had them uncheck me in and add my KTN into tomorrow's flight's PNR and then rechecked in & pre-check showed up on the boarding pass. Then I had to go to all future booked flights and add my KTN to each PNR to be sure it was there because I don't know when it disappeared from my profile - a real PIA. Now I intend to check my & my wife's profiles periodically to make sure the KTNs are there. I've never had this happen before and DL can't explain it (and all my flight are booked on DLs website).