Ford falcon front passenger door?

[tiger c]

Hi I have a ford falcon 1999 model forte. A few days ago suddenly when i locked the doors using the key lock it locks all doors except the front passenger side door. I have no idea why it does this and my mechanic says i will have to change something in that door which will cost me around AU$200. I find it wierd that when all the other doors lock then the one that doesnt should have a minor problem which the mechanic might be able to fix but wants me to pay a lot more for it. I know I am being skeptic but I am out of a job since a year so I am tryin to save some dough. Any guidance would be appreciated. Thanks