The ones we missed were Connecticut, Idaho, Louisiana, Mississippi, Montana, New Hampshire, Oklahoma, Pennsylvania, West Virginia and Wisconsin. Well, I didn't even know there _was_ a West Virginia - but the others, I just forgot.
We wondered if the ones we'd forgotten were clustered together. I'm pretty sure I know more about the coasts than the middle, for example. So I downloaded a map of the USA and coloured in the states according to whether we'd known them or not. Here it is:
Purple means that we both remembered it, blue means only Richard remembered it, green means only I remembered it and yellow states were the ones we both forgot.
Isn't that interesting? :)