Originally Posted by jerry a. laska

Just showing the card is not enough to provide access to the precheck lanes, you must have your KTN in your reservation and the TSA precheck indicator must be displayed on the boarding pass.

If adding it yourself doesn't work out then you need to go to a counter and have the agent make sure your KTN is in the correct field and issue you a paper boarding pass with the precheck indicator.
See also:
https://www.flyertalk.com/forum/alas...-precheck.html
https://www.flyertalk.com/forum/alas...ry-flight.html