Your Known Traveler Number is not attached to your MileagePlus number, it is part of your United profile. Only bookings made through United booking channels will take the KT number from your UA profile and put it into the record.
Travel agents must put the number in if they create the reservation. They put it in with the rest of the SecureFlight data.

THANK YOU! Your suggestion worked perfectly. Work travel agent put the number through, I reprinted the boarding pass, and I now have the magic number 3 for tomorrow.
Originally Posted by
villox
I think the question here is how can we get an agent to update the Secure Flight data after a reservation has been made, regardless of booking channel.
It was very easy with the previous interface on United.com.
I called our TA and the person I spoke with updated it immediately, despite the fact that I was checked in, and I "reprinted" my boarding pass a minute later and got the magic 3.
EDIT: Sorry, I misread your question. Still not sure how you get a United agent to update a United-booked ticket. Seems to me that if you edit it in your MP profile, it should go through since that's where United is pulling it from for your United profile?