Games : How to claim counties of my duchy?

I am playing as an English count of Northampton in 1070s. The English king has given me the Duchy of Wessex, but I don't have contol over the Wessex county. I can't declare war on Wessex: all the conditions are met other than that crown laws don't allow me to attack other counts of the kingdom. But I want Wessex. How do I get it under my domain?

And I don't understand what is the point of being able to become a Duke of Wessex if I can't have any of its counties.