Do you ever wonder why you can open a door handle with a credit card? It's because the door is not positioned correctly with the strike plate on the door frame. This could be due to a number of reasons. First, the strike plate might not be in the right place for the latch. In the photos to the left you will notice the latch and plate (the latch is the protruding object coming out of the door edge); the latch has two pieces--one bigger than the other. The smaller part of the latch is called the dead latch. If this is fully extended then the lock can be opened with a card, but if it is pushed in about 1/4" then you can't use a card. Test this by first pressing in the dead latch and then pushing on the latch (the bigger of the two)—the latch shouldn’t be able to move. Because of this, only the bigger part of the latch should go in the hole of the strike plate on the frame. This will leave the latch fully extended into the hole of the strike plate and the dead latch pressed in by the strike plate. If both go in then the lock can be opened by a card. Are you with me so far?
Another reason you might be able to open the door with a card is that the strike plate for the deadbolt is not positioned correctly on the frame. This is often the biggest issue with doors and can be easily fixed. A third reason a door might not lock correctly is that the weather stripping is worn out and allows the door to close too far; when you lock the door you have to push or pull the door into position to be locked and may not get a good seal on the door.
The last reason you might not be able to lock a door properly is mostly found in newly constructed homes where the builder has tried to square the frame and didn’t get it done quite right—it’s a matter of millimeters! This could also happen in older homes that have settled and the frame has shifted. The good news is that this is usually an easy fix with the correct knowledge and proper tools. Contact a locksmith to get a quote for checking all of your exterior doors and for proper installation of locks. They can also suggest any upgrades and new security ideas for your home.