I just installed a standard deadlatch, very much like this:
The problem is, it can be opened from the outside with a credit card.
Of course, a properly installed deadlatch should make this impossible. When the deadlatch is depressed, the latch should not be able to retract.
But when I hold the deadlatch down with my thumb and wiggle the latch, the latch eventually retracts.
It has nothing to do with how the strike plate is installed or positioned on the door. I can do it without ever involving the door or strike plate. I can just use my hands. It's very repeatable. Just hold the deadlatch down, press and wiggle the latch, and it retracts, every time.
Does this mean I have a faulty deadlatch? How could a deadlatch malfunction in this way?
UPDATE: I have to use a latching lock for this door. That's part of the users' requirements. So a deadbolt is not an option. Covering the gap between the door and frame might be a good idea for preventing the credit card move. However, what I'd really like to know is how a deadlatch could malfunction in this way to begin with.