You have something on your mind and you want to talk about it. While sharing your feelings, thoughts, etc. you realize he’s not paying attention. He’s tuned you out and his focus is elsewhere.
Your gut instinct will be to get upset and then talk about how rude it is that he tuned out.
What’s the point? Obviously something is on his mind or worse he has no interest in what you are saying. Unless you are doing something stupid like trying to have this conversation during a sports game you know he has an interest in (yeah, that’s stupid), he should be listening to you. If he isn’t, that is a sign. There is no point in talking to a wall.
Don’t try to force the issue. If it’s not a serious emergency, just chill and wait for another time. If you bring it up again and he tunes out, he doesn’t have an interest in what you’re saying…or you have the worst timing ever.