I am finding that now that I have 2 kids over 13 that failing to decline the optional UM service (or alternatively adding a link to an adult PNR) means that their KTN is in the system but not properly passed to whatever system to generate precheck. BTW- the younger (age 13) limit on Alaska is brilliant, my newly turned 13 year old jetted off to visit his cousin the day after he turned 13.