I assume it has the antiburst locks. (if not, ignore the following!)
These will sometimes fail to lock (either with the key or with the toggle inside) if the door is not closed properly - try with the door open and turn the bit that hits the striker plate by hand. If this is the problem, adjusting the striker plate so the door does not close quite as tightly may solve the problem, and some light oil squirted into the lock may help, as may just slamming the door harder.
Apart from these suggestions - new lock. Should be available from any parts supplier, last one I got came from Fred Smith in Melbourne, but that was years ago, and not handy to you. They are not particularly cheap.
John
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
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