Power steering problems

[Daggernite]

Hi, I'm having abit of a problem with my power steering. Basically every now and then, the power steering in my astra does'nt start working when I start the engine. Sometimes I have to start the engine multiple times before it activates again. I've checked for leaks and found nothing, I've topped off the power steering fluid, I've even waited a couple of week hoping it might fix itself. But its still happening.

Just wondering weather anyone can give me any advice or ideas as to what the problem can be before I have to go have it seen to.

Thanks