I had an intermittent problem with the door locks sometimes not locking.
One of my volvo mechanics said that might be to the the relays getting overheated due to the key lock not returning to its original position.

He suggested that when you insert and turn the key to open or close your door, that you rotate (guide) the key back to the original position (key back to vertical).
(He might have said to rotate it back until you get some resistance, which is about near vertical for me, and then take the key out).

I'm not sure why this is. Perhaps over time, a spring inside gets weak.

Anyways, I don't know about the internal intricate details, but I didn't seem to have any problems after that.