My 2000 S70 is inconsistent when unlocking The driver door usually unlocks but the others are hit and miss. This is my second S70 with the same problem. Usually happens most often when car is sitting in sun. I am in Arizona.

 

Had a similar problem, I changed two things, the locking system box, and the main switch assembly.

If everything else opens fine, like gas door and trunk, I would look into the main switches ( the ones on the drivers door) see if the plugs are clean and well connected, I just saw a how to on cleaning that part and making it work again.
After I did that, I went back and checked the actuators, one of them was funky, opened sometimes, and I changed it, now everything works fine.
I am no expert, just been fixing small things in my car by reading in forums and asking..
Hope this helps you

 

replace the actuator. My started acting up then it quit all together. Door was locked an shut. It would not open. Not a fun job. with the door stuck shut.