Every evening your father turns the deadbolt with the same little grunt, and lately it takes two tries and a shoulder against the door. You have noticed. He has noticed too, and he has decided it is fine, which is exactly the kind of decision that needs a second opinion from the person who is paying attention.
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Run the one test that tells you the cause
Close the door slowly and watch where the bolt meets the frame. Does it hit the hole in the strike plate, the metal piece screwed into the door jamb, or does it hit the wood just above or below? If the bolt lands cleanly in the hole but still drags, the problem is usually friction, and the fix is small. If the bolt clips the edge of the plate or hits wood, the door or frame has moved, and that is a different job.
The DIY version
If the strike plate is loose, tighten the screws. If the screw holes have stripped, pull the screws, fill the holes with wood glue and a few wooden toothpicks, let it cure, then drive the screws back in so they bite into the framing rather than the old hole. Slight misalignment can be eased by filing the edge of the hole a little larger with a metal file. Lubricate the bolt with a dry graphite product rather than oil, because oil collects dust and makes the problem worse within a season.
Do not try to fix a door that has visibly shifted, a cracked jamb, or a lock that has been damaged by force. If you need to pry or hammer to get it to move, stop.
When to hire it out
Call a handyman or a locksmith if the door sags at the hinge side, if there are cracks running from the strike plate into the wall, if the bolt has been damaged after a fall or a forced entry, or if you are not sure whether the lock itself is worn out. A locksmith can also re-key or replace the cylinder if you want to control who holds spare keys. Ask whether the visit includes the trip charge, whether it is a flat fee or hourly, and whether the work is guaranteed for a set time.
Before you decide
Think about who has keys. If a neighbor or a relative holds a spare, and the lock is getting sticky, this is a good moment to count them and decide who should keep one. A sticky deadbolt is also the moment a lock might fail when someone needs to get in quickly, so it is worth fixing soon even if the house feels calm.
What to do tonight
Stand at the door with the light on and watch the bolt go in and out three times. Note whether it catches the hole or the wood. Tighten any loose screws you can reach. If the test points to a shifted frame, make a phone call in the morning and tell the person on the line exactly what you saw, so they can bring the right tools the first time.