When you converse with one of us, and in response to something you've said, we begin to work on a task, probably impulsively, it is difficult for us to shift our attention back into an ordinary kind of conversation. This raises the issue of the facets of this disorder. Paying attention involves more than zooming-in on a topic and staying fixed on it, although that's very important, obviously. To interact effectively with our world and the various demands we all face, we must be able to zoom-out at will, too. That's tricky. Even with meds, my brain doesn't like repositioning itself. I want to be all in. If I have to shift my attention constantly, that's tough. I can do it, with meds, but it is difficult.
I said all that to say if your ADHDer seems to take off in the middle of having a talk with her, that could be a factor.
Also, if we seem inclined to shut you out, remember that we get frustrated too. 1. It isn't fun for anyone to fail repeatedly nor 2. to be reminded of it. Like everyone, we can only handle so much hurt and disappointment. Without meaning to, my SO does a heck of a job bombarding me with a tone in her voice that says, "You big idiot!" We get locked into ways of relating to one another that can be annoying due to years of conditioning. You know? I have to remind myself what hooking up with me was like for her pre-dx. She needs to remember that she doesn't have to tell me the same thing 63,000 times. I occasionally say, "I heard you the first time." (What a treat that is!)