[wozz]

When you try to lock are ALL of the actuators moving, (the one in the rear door?) but they bounce back and it doesn't lock?
The sensing afaik looks for a momentary pulse to ground on one of the wires, I don't know which wire, but you might be able to see it if you operate the locking with a voltmeter attached.
Does opening the door put your interior light on?

 

[wozz]

Lubricate anything that moves in the door, if it works sometimes it sounds like something is a bit sticky, so perhaps not making it far enough to cause contact. Don't go shorting random wires to ground, it will end badly.

 

[wozz]

You didn't say it had a sliding door, it's a common fault that the connections go faulty, there are lots of posts about this.
Clean the connectors and make sure the pins slide freely, also check wiring behind connectors for corrosion. You may need to tweak things to align the connections
(That doesn't tally with the tried 5 or 10 times and it worked unless you moved the sliding door)

 

[wozz]

I'm no expert...and apologies I don't have a wiring diagram.
Is the sliding door locking? If the sliding door is working (locking) those contacts are OK
Can you hear any noise slight click (anything) from the rear doors lock at all when the C. locking is operated?

The locks do fail, <- have you worked it back and forth manually? (The lack of voltage might be a red herring depending on where/how you measured).