
He failed his top-level clearance test.
Even though he's a tough-as-nails agent, it was revealed on "Citadel" (Season 7, Episode 2) that Callen actually previously failed his HL7, which is a behavioral test for top-secret clearance.
Watch NCIS: Los Angeles on Sundays at 8/7c on CBS and CBS All Access.
Watch NCIS: Los Angeles on Sundays at 8/7c on CBS and CBS All Access.