Your memory could be playing a part. Maybe you have trouble remembering details, which is essential for retelling a story. It could also be that you're not used to speaking in a narrative way. If you don't often tell stories, it can feel awkward and difficult. You need to train your brain to think in terms of a story's structure, starting with a beginning, building up to a middle with a problem, and then reaching an end with a solution.
It might be difficult because you're not fully understanding the story in the first place. If you don't have a clear grasp of the plot, characters, and their motives, it'll be tough to retell. Another reason could be lack of practice. Just like any skill, retelling stories takes practice.