Hmm. Have you tried to save and reload?
What is your Sov's Chr, and does he/she or your faction have any traits that make hiring champions harder?
There is a flaw, I think, in the tech tree. Sometimes you can't hire champions or marry children based on what techs you have researched, and it can be strange. If you have researched nothing, you can hire and marry just fine, but get into some strange middle zone where you have researched just a little, then you can't.
