How do I fix the locks on my 2003 Astra?

When I lock my Astra the doors will make their clicking noise twice and two will be locked and one unlocked- or vice versa. I'll then press the unlock button, however nothing happens until I press lock- which somehow unlocks the car.
I've checked the manual and I'm pressing the right buttons.
I've also locked the car manually from the driver's door as per. the owner's manual, but it'll only lock and unlock the driver door.

What is causing this problem and is it fixable?
I'll be grateful for any assistance!
If its of use- its a Holden Astra SXi and I live in Melbourne's South-East.