Dad says the deadbolt works. He is right. The lock is fine. The problem is the strike plate, the metal piece on the door frame that the bolt slides into, and the wood behind it. A lock that engages cleanly can still give way when someone leans on the door, because the bolt is only holding as firmly as the screws holding the plate to the frame.
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What to check tonight
Close the door, turn the deadbolt, and look at the plate. Ask these questions in order.
Are the screws the short ones that came packed with the plate? Those screws often only bite into the door jamb, not the solid framing behind it. If you can see that the screws are short and the plate is sitting on thin trim, the hold is weak no matter how good the lock is.
Is the wood split, crushed, or soft around the hole? Press a screwdriver tip into the wood near the plate. If it sinks in or the grain has torn, the screws have nothing to grab.
Does the bolt line up with the hole? Close the door slowly and watch whether the bolt hits the plate square or rides against the edge. If the door sags and the bolt catches high or low, the problem is the door, not the plate.
Before you touch anything, confirm that the door can still be opened from the inside without a key and that whoever lives there knows how. A fix that traps someone is worse than the one you started with.
The do-it-yourself version
If the wood is solid and the only problem is short screws, the fix is ordinary. Remove the plate, replace the screws with longer ones that reach the framing behind the jamb, and drive them snug without stripping the heads. Check the length against the depth of the frame before you buy. If the bolt drags after the new screws go in, the plate has moved, and you need to loosen it, shift it, and re-test before you tighten it all the way.
Use the manufacturer's installation sheet for the lock and plate if you have it. Those sheets list the screw length and the spot where the plate belongs. The sheet is the source that matters here, not a rule of thumb from a neighbor.
When to call a carpenter
Stop and hire the job if any of these are true. The jamb is split, rotted, or soft through its depth. The door sags enough that the bolt misses the plate even after adjustment. The plate hole has been enlarged by a previous kick-in and needs a wood filler or a new section of frame. Any of those is a frame repair, and a frame repair is not a screw swap.
If you hire it, ask what the scope covers. Find out whether the quote replaces only the plate and screws or also repairs the jamb and reinforces the framing. Ask whether they will check the stud depth and tell you what they found. Ask them to leave the door locking and unlocking properly, tested with the key and from the inside, before they leave. If you need a name, start with Local Help in St. Louis.
Your next move
Tonight, close the side door, turn the deadbolt, and push on the door from the outside. Note whether the bolt stays put and whether the plate moves. Then remove one strike screw and look at the wood it came out of. Take a photo of the hole and the screw, and send it to whoever will do the work. A clear photo gets you a clear answer faster than a description, and it tells you whether this is a screw job or a frame job before anyone picks up a drill.