Sometimes that happens with my truck, and I just spray a little WD 40 into the door lock mechanism using that thin red tube that comes with a can of WD 40. If that doesn’t work, I’ll spray some WD 40 on the key, and then put the key into the lock and work it around. Sometimes it takes two or three days of doing this, then it will loosen up.
Other times all that doesn’t help and I have to remove the inside door panel for a look-see to find what all is binding. Often it is a screw or nut that has come loose. One time I had to replace the lock cylinder, which meant replacing three lock cylinders, two for the doors, one for the ignition, since I wanted the same key for all three. That was a bit of a pain.
I think on a 99 model you are safer to only use a dry lock lubricant, not WD 40. I’m just saying what I use. My truck in 40 years old, so I’m ok with having limited expectations.