Most likely the lubricant (grease or whatever) in the lock and at linkage mechanism has either dried up or gummed up. Or, there's some corrosion that prevent the lock and linkage to operate smoothly. It might help for a short period if you spray some lubricant at the lock and the inside linkage. The better fix (IMO) is to take the lock apart, clean and re-lubricate it. Especially for those who live in cold areas, moisture that get inside the lock will increase the possibility that the lock will get frozen at the worst time of the year. Here's my writeup if you like to read about my experience.