Could be a bunch of things, but it’s really not designed to stay closed unless the arm is down, which drives the pin backward into the frame.
The little spring thing and closing the door without putting the arm down, you can pop most Cessna doors open by flexing the airframe by wiggling the wings. Even when adjusted correctly fresh from the factory.
Are you concerned about it for security or for wind? For wind, I can see trying to get them to behave. For security, there’s only about five keys for all Cessnas built before the restarts in the late 90s and I know mechanics and others with a full set on a key ring.
The way to secure a Cessna is to put it inside a hangar. The doors aren’t truly secure.
Often if they won’t latch and the mechanism is intact, the entire door has been sprung out of place or bent by a wind event catching the door and bending it and it’ll have to be adjusted and maybe re-shimmed depending on how much it’s warped. Also extra layers of paint often make this problematic, which affects our airplane a bit. It wasn’t stripped prior to the new paint job in the early 90s, so the doors can be a tad of a pain.
Cardinal doors are very vulnerable to being left open and hit by winds because of their size and relatively smaller attachment lines. A wind-bent Cardinal door becomes a natural pain in the butt until someone repairs it.
