Wow, thanks everyone for answering. I'm sorry to hear some of these awful and frustrating stories, but I really appreciate you telling them and (/or) doing the poll.
I never thought about parents who tried to learn but just couldn't get it. Learning about them should increase my tolerance and patience a little. I hope! (That's always very easy to say and very hard to do.)
This is also making me wonder about the ones who don't try. I always imagined people with attitude problems or who'd been brainwashed by the oralists. But I wonder if some of them, if they tried, would have ended up in the ""tried but couldn't get it" group. And on some level had a sense of that. I say this because when I think of my own strengths and weaknesses (I'm very good at math and learning languages, but I think if I tried to learn to fix a car from now until the day I die I'd never get it. And when I try to watch the news I feel like someone's drilling a hole through my head.) I find that my interests often line up with my abilities. If a mathematical idea comes up in conversation, my attention perks up. When I first saw people signing I was so blown away I think I tripped over something. But if you try to show me how spark plugs work, I start to feel nauseous and at some point I realize I stopped paying attention 5 minutes ago. Didn't mean to!