Locking a door seems simple, but the mechanism that does the work is complicated, which leaves it susceptible to jamming. Freezing weather can cause the locks to jam, and moisture may cause the tumblers to rust and become stuck. If the lock mechanism is working, but it is still hard to turn the key, the deadbolt may be out of alignment with the strike plate on the jamb. None of these problems are serious, and they can all be fixed with a few hand tools and some lubricant.
Cold or damp weather can mess up the door lock mechanism, causing it to jam. Before you blame the weather, make sure you are using the right key. It seems obvious, but it's an easy mistake to make. If it's a cold day and the key won't go into the lock or you can't turn it, heat it by blowing on it before trying again. If the lock is really frozen, a hairdryer can help thaw it out, so it stops sticking.
Dealing With Burrs
If warming up the key in cold weather doesn't work, or you can't get the lock to turn in warm weather, the key may have burrs, especially if it is a new one. Spread a little soot on the key, insert it in the lock, and remove it. Any parts of the key that don't have soot are rubbing against the mechanism. Grind those areas down with a file, and try the key again.
Lubricating the Lock
A little grease may do the trick if your deadbolt is sticking. Spray thread lubricant into the keyhole if the deadbolt is difficult to turn when the door is open. This will loosen deposits on the metal parts that are preventing the key from turning. Keep a little spray lubricant in the garage, especially if you are having problems with your lock, so you don't get locked out of the house. You can also squeeze a little graphite into the hole, or dust some on the key. Then insert the key, and turn it a few times to work the graphite into the mechanism.
If the parts of the deadbolt don't line up correctly, it's difficult for the mechanism to work properly. Check the alignment of the deadbolt against the strike on the jamb if the lock is difficult to turn when the door is closed or the door doesn't lock at all. Use a file to widen the hole on the strike, or remove the strike with a screwdriver and reposition it on the jamb. You may have to widen the mortise in the wood under the strike when if you reposition it. Use a chisel to do this.
Remove the cover plate and tighten the set-screws holding the lockset together if it moves when you turn the key. If either of the screws is broken, replace it. Disassemble and remove the entire lockset if the key is still hard to turn. Spray the entire mechanism with spray lubricant and wait for at least 10 minutes before you reinstall it.
Sometimes the cause of a stuck deadbolt is a misaligned or out-of-square door. If the gap between the door and jamb is angled, then the door isn't square. Adjust the hinges so that the door is straight.