It can also be a worn key.
My car came with two keys in 1995 - I guess the door locks had been replaced after a breakin or something. Anyway, I had one key for the doors and another for the ignition.
However, some years down the line (the last 5-8 years) the doors have stopped accepting the original key, but rather are now happy with the original ignition key. ??
They obviously have a life of their own, - but now my car only needs one key.
PS: I keep the old door-key in case my ignitionlock suddenly plays up
/Lennart