This is bug that needs to be fixed in XsdBasedSoap11Wsdl4jDefinitionBuilder according to Arjen Poutsma. The workaround is to expose all xsd files. This bug needs to be fixed so that we can design and build modular web services.
There is a detail description about this issue on Spring Web Service Forum. The thread is created on "How spring is handling multiple xsds in generating a WSDL file"?