Hello and welcome to JustAnswer!
It sounds like you have a mechanical failure of the latch itself. If the lock button pops up when using the power locks then you know the system is electrically ok. The problem has to be with the internal linkage in the door latch and replacing the latch should take care of the issue.
I can try that later this week - need to make time to take panels off etc.
But why on every second attempt does it functions OK?
If sometimes it was on the first attempt, sometimes second or even third attempt I could understand it but not perfect every second opening/locking cycle.
What do you mean by the latch - is this the external handle or simply the total mechanical movement?
I think that you are correct- I had to travel across to South Durban on business this morning and had time to call in at the dealer.
They told me that the actuator needs replacement - demonstrated by showing me the ease of moving the open/close button up and down - almost no resistance.
So will complete process for you now and obtain a replacement actuator sometime soon.