Lately the Power Lock button on the driver's armrest is not working consistenly. When I get into my car and close the door, the first thing I do is press the lock button, but about 90% of the time nothing happens. I'm not convinced that the button is physically broken because if I press the unlock button first and then the lock button, the lock button will always work. (fyi, I've never modified the locking and unlocking options described in the driver's manual so they are still set to the default settings).

Has anyone else noticed a problem like this before?