I have similar problem happened once or twice. The driver or the passanger door stops doing its job, and the other door works fine. I used to disconnect the fuse and connect it back while doors are unlooked. This resolves the problem. My interperetation is that, the central lock pump has a transistor (brain) telling it to ignore a problematic door and keep the other.However, I cannot say how this is translated into lower voltage recived? I assume that there is some rusted or not well connected things in the door locking mechanism itself. and the pump when not receiving confirmation assumes problem. disconnecting the fuse resets the pump. If you connect everything and reset using the fuse, it will work again. that is only my theory.

very problematic system!!