Things makes sense once they make sense, so it can be hard to remember why they didn't. We build complex networks of understanding and forget how intricate the path to our available knowledge really is. This bias is closely related to the hindsight bias wherein you will tend to believe that an event was predictable all along once it has occurred. We have difficulty reconstructing our own prior mental states of confusion and ignorance once we have clear knowledge.
When teaching someone something new, go slow and explain like they're ten years old (without being patronizing). Repeat key points and facilitate active practice to help embed knowledge.