Here are some symptoms of possible issues that you may encounter with your Door Locks:

 

Key Gets Stuck / Hard To Turn In Lock:
The first and most obvious step is to be sure you're using the right key. Then open the door and try the key again. If it works easily, the dead bolt isn't engaging the strike plate properly. If it doesn't work any easier, lubricate and/or clean the lock. Spray a little graphite into the cylinder and try the key several times. If this doesn't help, consider calling a locksmith.

Key Turns but Doesn't Work
Disassemble the lock so that you can be sure the cam or tang in properly engaged with the bolt. Re-engage it if necessary. Replace any broken parts and reassemble the lock.

Key Won't Go In Lock
Is the weather cold enough for the lock to be frozen? If so, heat the key and, wearing gloves, insert it gradually into the keyway. Repeat heating and inserting until the ice has melted. If the lock isn't frozen and you're using an old key, sometimes squirting a little graphite powder into the lockset will do the trick. If need be, have a new replacement key made by a locksmith.

A new key that won't go in or work properly may have rough spots that need to be filed off. To find them, hold the key over a candle to blacken it with soot, then turn it very slightly in the lock and remove it. File down shiny areas where the soot was removed by the unwanted ridges.

Door Lock Works Slowly
Exterior locks can freeze, interior locks get dirty, and small internal parts eventually wear out or break. Before you buy a replacement, try some quick remedies: Put some graphite into the keyhole, either by squeezing it from a tube or dusted onto a key, then operate the lock a few times to work the graphite into the mechanism.

Lock de-icers contain alcohol and other lubricants that help to dissolve gummy, dirty deposits. The last resort is to disassemble the lock to see if something has jammed or broken-you may be able to set it straight or replace the part without buying a whole new lock.

Entire Lock Cylinder Turns
A cylinder turns when the set screw(s) meant to hold it in place is loose or broken.

Mortise lockset: Remove the faceplate (if there is one) at the door's edge and locate the one or two cylinder set screws. They should be in line with the center of the cylinder. Tighten by turning clockwise--be sure they engage the slot that runs along the edge of the cylinder (the key slot should be perfectly vertical). Replace the faceplate.

Surface-mounted rim lock: Unscrew and remove the cover, called a "case." Tighten the cylinder set screws. Replace the case.

Door Lock Doesn't Latch Shut
When a door latch doesn't click into position, it usually means the latch and the strike plate are out of alignment. Tighten its hinge screws, then try adjusting the strike plate by loosening its screws and shifting it slightly.

When possible, it's easier to file the slot in the strike plate a little bit so that it will receive the latch. Shifting the strike plate's position usually involves mortising the jamb, filling part of the old mortise, and so forth. You can also solve misalignment by replacing the strike plate with an adjustable one.

How to Fix a Stuck Door Latch
A latch can stick for many reasons, most of which are easily fixed. Check that the hinge screws are tight. If the door is out of alignment, the latch will bind. Also check the knob and lock assembly for loose screws or misalignment. Finally, look closely at the strike on the door jamb-if it's blocked or out of adjustment, the latch won't run freely in and out.

How to Fix a Stuck Dead Bolt
The chances are good that the bolt is having a hard time finding the throat in the strike plate. Be sure the strike plate is secure and is in reasonable alignment with the bolt. You can file the edges of the strike plate a little, and even slightly round the edges of the deadbolt's end. If this doesn't work, you'll probably have to remove the strike plate, fill the screw holes with glue and wood matchsticks, reposition it properly, and re-screw it.