1. I honestly don't know it as well as I think. This is a hard pill to swallow, but most of the time it's the reason why we struggle to explain things in clear terms.
2. We are often conditioned to use terms that the audience doesn't fully understand. e.g., Industry terms.
3. I haven't given it enough thought before trying to explain it.
Find someone who is willing to listen to you speak. Hopefully they'll be patient enough to listen to you when you repeat yourself in different ways.
Lastly, I found that sometimes I struggle with trying to over explain things. There's a tipping point where you're audience is "comfortable" with what you explained, but once you go beyond that, it's like dragging them into deep waters.
Learn to be comfortable with not being fully "correct". Meaning, explain enough so they have a good idea of what you're saying. It's a starting point for the next thing they will need/want to know.