What's usually wrong with door locks? Although there are variations depending on the type of the lock, it's most likely that the latch, strike plate or cylinder is damaged. Whether the problem is fixable or not depends on the extent of damage. Sometimes, lock repair is necessary because the door doesn't close very well or the latch is not inserted completely into the strike hole. In other cases, the problem is worse and often related to broken parts of the mechanism. In this occasion, repair won't do much good. The broken parts would not function well and your security is at stake. In such cases, customers need to have these particular locks replaced.
* Strike plate problems: the strike plate is found at the door jamb. It is made of steel and if it's distorted, it cannot be fixed properly. When there is damage, the door won't be able to close perfectly well and requires replacement. Though, the most common problems with strike plates are related to their alignment with the latch. Loose screws misalign the strike plate and this would be a problem. So, check its condition and untighten its screws. Make sure the strike plate takes its initial position before you tighten the screws again. Then, check that the latch of your door lockset finds the hole and secures well.
* Latches: they are parts of your security door locks. Their size and quality depends on the bolt and are usually replaced along with the whole lock. If they are seriously damaged or even broken, you must get a new one. As far as their maintenance is concerned, good lubrication is essential in order for them to move smoother and slide into the strike's hole all the way. If the latch is not aligned with the strike plate but the latter is in place, see if there is something wrong with the mechanism. You must unscrew the doorknob screws and remove the cylinder. A broken mechanism cannot really be fixed and lock replacement would be required. If the cylinder is only loose, tighten it. This should align the latch.
* Cylinder: it's the main part of the lock system. If you need lock rekey, replace its pins. The cylinder contains the codes which determine whether the door will lock or unlock. In order to rekey it, purchase a kit for your own brand and use tweezers in order to insert the new pins in accordance to the colors of the codes. If it's loose, you need to tighten its setscrews clockwise. For mortice locks, make sure the key slot remains in vertical position. If the cylinder is loose, the key would just turn in the lock and will neither lock or unlock the door.