Why a Key Starts to Stick
A key that has to be jiggled or forced is telling you something. It is usually not the lock alone.
The door has moved
Houses settle, hinges wear, and weather swells wood. When a door drops or shifts even slightly, the deadbolt no longer lines up with the hole in the strike plate. The key then has to force the bolt against the metal edge, which feels exactly like a failing lock. A tell-tale sign: it turns easily when the door is open, and only sticks when the door is shut.
The key is worn
Keys are brass and brass is soft. A key carried for years, especially a frequently used copy of a copy, wears down at the peaks. Eventually it no longer lifts the pins to the right heights and it drags or catches. A freshly cut key from a good original often fixes this on its own.
Debris or dry lubrication in the cylinder
Pocket lint, dust, and old oily lubricant that has gone gummy all collect in the keyway. The pins stop moving freely. A proper cleaning and the right dry lubricant usually restore smooth operation. Household oil is not the right product here; it attracts more grit.
Wear inside the lock
Springs weaken, pins and the plug wear, and after enough cycles the tolerances that made the lock smooth are gone. At this point the cylinder itself needs servicing or replacing.
Why forcing it is the wrong move
Every hard turn twists the blade. Brass fatigues, and one day it snaps, usually with half the key inside the cylinder. Now a simple service call has become an extraction plus a new key. If a key is getting stiff, treat it as a short countdown and have it looked at while it still works.
The lock repair page covers how the cause is diagnosed. To have a sticking lock checked, call (866) 306-9624.