Why won't my Astra start?

[missaj07]

I Have a 2006 Holden Astra CD AH and it has a starting problem.

Rarely my car will not start, It seems to be something to do with the ignition or its computer.
When this happens it makes a clicking noise when i try to kick it over, sometimes after about 5 minutes it will kick over, other times not at all. In that case we can push start it and after that it runs fine and will kick over again like nothing was wrong until a few weeks later whenever it decides to do it again. What could be happening?
I've also realised it does it once the battery is 12.6V or under (12.6V is a good battery and should definitely kick it over) so we can also jump start it once its past 12.6V .
Has anyone else had or heard of this problem?
I have had the battery checked and starter motor checked and cleaned up because signs made us think it was the starter motor.
The car still randomly won't start!