Hi vassilios,
I've got four Kribs of my own, but started with only three. I had experienced the same problems as yourself, the female not wanting to do anything with the male, or any other Krib.
Its all about size. If your male is much larger than the female, then she'll give him a lot of respect, and if you're really lucky she'll want to breed with him. But, until then it may be better to seperate the two of them, as one of them may die as a result from the aggression shown from the female.
Also you might need to provide as many hiding places as possible for the male to hide in.
Keep feeding them, the male will get a lot bigger than the female.