Doorknobs stick for several reasons, including loose or missing setscrews and a lack of lubrication. Determining the cause of the malfunction sometimes takes some investigative work, but it is often possible to make repairs without having to replace the lockset.
The doorknob attaches to a rod that extends through the lock and turns the knob. A small screw usually attaches the knob to the rod. If the screw is loose, the knob sometimes turns without moving the rod. To fix the problem, the owner should locate the screw and use the appropriate tool to tighten it so the knob works correctly once again.
Locks are mechanical mechanisms that sometimes require lubrication. Most hardware stores sell graphite lock lubricants. Unlike petroleum-based lubricants, graphite does not evaporate over time so it eliminates any sticky residue. If the lock is sticking because someone in the past used a petroleum-based lubricant to oil the lock, lock deicers include an alcohol-based solvent that helps to remove the sticky residue.
As buildings settle, the lock and strike plate sometimes become misaligned. As a result, the latch does not catch. It is sometimes possible to move the strike plate higher or lower to match the lock. If the misalignment is more than 1/8 inch, it is often necessary to remove the plate and enlarge the hole to move it to the proper location.