Hi James, it's one of two things! either both the door locks are worn (very common) and they will only lock not unlock or also very common a little spring inside the latch/lock assembly falls off or breaks and you get the same problem which unless you catch it early will lead to wear in the lock (key) assembly only working in the lock mode, If you would like more details contact me on nige110162@hotmail.co.uk
nigel, August 2008