I assume you're talking about power door locks. I don't have PDL, but I was having issues with my manual door lock not locking, which turned out to be a problem with the door latch mechanism. I had a big rust hole in the bottom edge of the door, which, of course, allowed water to get in and mess up the latch mechanism. I was able to get it out to thoroughly clean and lube it, which temporarily solved my problem until I replaced the entire door.
Sounds like a bad switch, or dirty contacts in the switch. The way those work is they receive power through a single wire buss. Power in one direction is full voltage while the other direction it is run through a resistor. The lower voltage from the resistor will cause the CTM to reverse direction. At least that is how I believe it works. Is the problem just on one door? If so possibly you could try using the opposie side switch to double check this.