I was in a similar situation when I first received my KTN. Had two existing reservations and called WN to get them added to my reservations and profile. Checked my boarding pass day before flight and it didn't appear. Called WN, had them drop the number and re-add (as others have said to do), but it still didn't work.

Same deal for second existing reservation.

Many here have said that you should get it if you add the number after the fact, but it didn't work for me. Tried every work around and spent time talking to the WN folks on the phone... to no avail.

Only starting getting precheck for reservations booked after adding the KTN to my WN profile.

Maybe you'll have better luck.