I can't lock my car with a key outside...

My car is Altima GXE, 1999. I used to open and lock with my master key. But I couldn't have lock my car outside since a few days ago.
So, I have to lock with the door lock knob inside the vehicle. I can still open it with my key. What's the problem?
Thanks in advance!
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Hiya! This is a common occurrence for me- I have a 98 Sentra XE. I usually can't use the passenger door lock or the trunk lock with my key, so I understand your trauma. What I've done is gone to Advance Auto and purchased a can of aerosol spray silicon- the kind that comes with the little straw for spraying guidance. I insert the straw into the keyhole and spray. The silicon lubricates the locks and lets me use the key again. I do this every few months in each lock and it helps a lot. This is because the original grease or whatever it is in the locks has worn off and the silicon re-lubes it. The spray is $2-$4 and if it doesn't work, you didn't lose a lot of money. I know it sounds ghetto but it worked for me :-)
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