Johnny's hearing impaired but still has selective hearing.
I do understand your frustration.
Have you tried giving her enough time to respond to the command as well as giving a simple, one step command in a low and firm voice without a lot of extra "wordiness" in the order? I think I got that from Susan People's book "How Children With Down Syndrome Learn". (I think that is the title...something like that)
For example: Instead of saying "Johnny, do your pants need changed? Are you wet? Stand up, go get a diaper, and bring it to Mommy" I need to totally cut out all the extra sentences and words and say, "Bring Mommy a diaper". Then let it sink in for at LEAST 30 seconds to one full min. or so. I may need to repeat the exact same words a couple of times. It's not that he's being bad, not listening, or disobeying (sometimes

) but he is processing what I said, what he's heard, what he's going to do, and then how he's going to do it before he brings the diaper to me. That's a lot going on!
NOW, that said, there are times I tell him something and he will look at me, stand up and bolt high speed into the other room and shut the door behind himself laughing. That is just a three year old thing....

Johnny's Journal:
http://www.caringbridge.org/oh/johncamden/index.htm