We had the same experience as FlyerJ recently in SJU.
Check-in agent was able to enter KTN, but wouldn't reprint boarding passes. We were told that WestJet printers simply could not print the TSA PreCheck flag on boarding passes, and we should show our nexus cards to access the precheck line.
TSAPre-check gate guard wouldn't accept Nexus cards, and we wound up in the general screening line, with 2500 other travelers who just disembarked from 2 cruise ships.