There are things that can interfere with the correct operation of a lock that have nothing to do with the lock itself. A couple of the most common are poorly cut duplicate keys and sagging doors.
Duplicate Keys : Hardware store key duplication machines are often not adjusted correctly. A few thousandths of an inch can make the difference whether a key will work or not. It is difficult for a customer to tell if the depth of cut is correct without using a micrometer to compare an original to a duplicate key. However, it is often very easy to detect improper spacing on a duplicate key. If a key won't turn when fully inserted into the lock, but will turn if the key is pulled out slightly, you have a miscut key - the spacing is off. But the quickest way to detect a spacing error is to look at the tip of the key. The ramp at the tip end of the key should be smooth. If it looks like a small notch is cut into the key on that front ramp section, it is likely the spacing is off.
If you look closely at the key in the picture on the left, you will see a small notch cut into the angled ramp section at the tip of the key - a good sign that the key duplicating machine is not properly adjusted, and that the key will not work properly, if at all. This is a very, very common problem.
And, yes, we have had a couple customers call us back after we have serviced a lock, claiming the lock was no longer working. Sure enough, they'll hand us a key duplicated at a local hardware, like the one in the photo. Better to pay us for all the copies of the keys you need when we service the lock than to pay us for a second service call when your hardware store copies don't work!
Sagging Doors : Many times, what seems like a lock problem is actually a door problem. A new lock will not fix a door problem, at least not permanently. First, make sure all hinge screws have been installed. Most hinges are designed to be installed with four screws fastening the hinge to the door and four screws fastening the hinge to the jamb. It's amazing how many times we have found hinges installed with as few as two screws on each side, even in newly constructed homes. Second, remove a couple screws from the hinge side. They should be at least 1-1/2 inches long, preferably 3 inches to secure the hinge to the wall framing surrounding the door opening. Don't be surprised, though, if you find 3/4 inch screws. Replace these with longer screws, and make sure all screws are tight. You'll often find that the short screws have stripped the wood and will continue to turn without tightening. Inadequate hinge installation allows the door to sag, and the locks will no longer be aligned correctly with the strikes. Also, look at the gap between the door and the jamb on all sides. The gap should be consistent, not wider at the top than at the bottom. A proper gap is approximately 1/8 inch on all sides.
Altic Lock Service recommends installing the Door Devil reinforcement kit on all doors. Part of the kit includes the longer hinge attachment screws to tie the hinges into the wall framing. This, alone, will often correct strike alignment problems.
Remember that most locksmiths can address most door hardware issues, not just locks; but also keep in mind that not all lock problems are caused by the lock!