I believe a significant cultural shift relates to organisations who were once competitors are merged into a a single entity working toward common goals. Requires expert leadership to achieve successfully.
I think Culture shifts speak to how things are done in an org- Schein’s model. These can be written and or understood. An employee who doesn’t understand the impact of the change and their place in that change will see now reason to adopt the new process negating any shift.
One cultural shift challenge I observed was how decisions are made. I worked for a company in which input on decisions was solicited from various levels, not quite a bottom-up/grass roots structure, but leaning toward it. This company was acquired by a company that was very top-down and hierarchical. Decisions were made up top and then communicated down without any prior discussion. This caused a lot of friction during integration. Employees who were used to being asked for input were suddenly kept out of conversations completely. It was certainly a culture shift.
I believe some people in the organization had resistance to change. A change in culture isn’t easy and can do more harm than good if not implemented properly.
Besides communication, the challenge is that the change process is not clear. If the process of change is not known or clear to people, they would be anxious and have questions in their minds. They may question their ability to adapt and belong, it may bring insecurity of how they fit in, do they have the required skills, whether will it increase their workload, etc.