I have been working with SSIS using the XML Task and XML Source Adapter. I was getting an error say that an element is was not defined. When I check the XSD, which was using import / include schemas, I found the element was difined but not in the main XSD schema the adapter was pointing to but another schema which was being imported. The probelm was with the schema were the element was defined not being invaild for the XML Source Adapter, that was the real probelm, as soon as I corrected the imported schema the error was resolved.
The probelm with the imported XSD schema there was some DTD define at the being of the file and XML Source Adapter does not support a mixture of DTD and XSD.