Locksmiths have long used powdered graphite to lubricate the workings of locks, and if your key refuses to slide fully into the lock, you can do the same. But there’s an easier (and less expensive) option than going out and buying a tube of powdered graphite. All you need is a pencil.
Simply rub the teeth of the key with a sharp pencil until the surface is covered with graphite. Don’t be shy, load it up with a thick coat! Then insert the key into the lock again, which will deposit the graphite into the lock. Do this several times, if needed, until the key glides in smoothly.