I usually had problems in the winter unlocking my doors. I live in the part of the US that gets pretty cold in the winter. Sometimes, moisture gets between your door and can freeze the lock linkage. I was always able to luck my door, but it would never fully unlock. I would have to use my key and keep turning it while I lifted the handle.

It is possible you have a bad solenoid that is not getting good contact when you try to lock it. Since your car is a 2000, you're probably still under warranty. I would recommend bringing your car to the dealership to have it checked out. If that lock linkage is still covered, get it fixed. If the alarm you have in that car was installed after you bought the car by someone other than the dealership, take your car to where ever you got it installed.

Good luck!