Most locking faults are alignment or gearbox issues, and both are identifiable in minutes.
When you have to lift the handle hard, or lean on the door to turn the key, the door is telling you something specific. Ignoring it usually ends with a gearbox failure at the worst possible moment.
Close the door and look at the gap down the lock side. If it tapers, the door has dropped and the lock hooks are no longer meeting the keeps cleanly. Hinge adjustment often restores it.
A grinding or free-spinning handle points to a worn multipoint gearbox. These are replaceable in most cases without changing the door, provided the mechanism is still available.
Doors move slightly with temperature, so a fault that appears only in summer or winter usually indicates the alignment is marginal. A stiff key alone is more likely a cylinder or lubrication issue than a lock failure.
Describe the symptom and we'll tell you whether it's an adjustment, a mechanism or a new door.
If the slab and frame are sound, replacing the mechanism or adjusting the hinges is far cheaper than a new door.
If the door is older, an anti-snap cylinder to current standards is a sensible, low-cost upgrade.
Warped slabs, failed welds, rotted timber sub-frames or obsolete hardware are the usual triggers.