Can I see it? Can you send me how to look at your admin?
I have a feeling it's one of two things:
1) you got the subcategory name wrong -- some minor difference between it and the parent
or
2) there's a permissions problem and you are not being notified of it for some reason.
I can't figure out any easy way to try to tell you how to fix it, other than the above. I'm curious, because this particular problem has not been described before (at least this way).