I also have this problem and have found a fix. The same thing happened to my front door and was able to fix the same way. The problem is the plastic gears in the lock actuator has one or more broken teeth. Any repair place will tell you to buy a whole new latch assembly which has the lock actuator inside of it (3-400 dollars). If you are mechanical, you may buy the gear itself for around 15 dollars from ebay. You will have to carefully take off the latch assembly, open it up by prying back the metal tabs, this will separate the outer casing of it. Then take the screws out of the black part that the wires go into and carefully pry off the top. You will then have access to the gear to change. I took mine apart first to make sure I was buying the right gear from ebay. Good Luck.
