subdomain of a subdomain

I have installed mobile joomla on my root, which works fine, but I have a subdomain with another site and I am trying to work it with that subdomain as well. When I create my subdirectory either at the root of my subdomain, or at the root of my domain, even though I type in the address of my subdomain (ie iphone.subdomain.com) it goes instead to the root. Is there a simple explanation for fixing this? much appreciation if someone can help.

Bob