This is the short version of what you should attempt to do before trying anything else :) If the screw is loose the key will not turn all the way and the turn button inside will not either.

If you are interested in more info please check out the longer version of this video that covers removing lever from door and taking apart    • Kwikset Series 5: Smartkey Lever Handles  
This video is a portion of that video, so if you watched that video there is no real need to watch this video. Unless you want to, you certainly can :)