What challenges leaders and HR professionals working with organizational culture are two things:
Firstly, it's difficult to spot the culture in the same way that a fish can't spot the water it swims in. Secondly, it's difficult to push the culture in a certain direction 'from the outside'. It is the culture's own key people (formal and informal leaders/culture bearers) who need to mobilize. You can't change the culture without involving the wider community in an analysis of the existing culture and a description of the desired culture.