My cockatiels not mating they are little bit fighting?

First clipping there flight feathers is cruel birds are meant to flight. It helps them keep both physical and mental fit. It is so important if they are keep in a cage they get safe daily free flight. As for breeding caged birds often won't breed . To breed they have to be happy and everything has to be just right. It sounds like you have avairy breed birds which are hard and usual impossible to tame. If you wanted tame you should have bought handreared birds.