What does a trap primer do when it fails?

Answer 1

You know a trap primer has failed or is not working properly if the drain dries out and begins to smell like sewer gas. Sometimes the primer can be adjusted with an Allen wrench to get the water dripping again. Turn the Allen nut counterclockwise until you can see water dripping through the airgap.

In my opinion as a journeyman plumber, they are notorious for failing over short periods of time.