Samum, now that we know that the tree is a seedling tree, the reasoning becomes clear why your tree has not flowered. Citrus have a very long juvenile period, normally lasting anywhere from 6 to 10 years. Until the tree' has grown high enough to reach maturity the tree cannot flower, thus does not fruit. Even then, the tree will flower only from the point where maturity started and above. The portion of the tree below the maturity point will forever remain immature.