What does your code look like that gets the schema manager? Here is mine:
BXN::XmlInstanceSchemaManager schemaManager;
bool status;
schemaManager = BXN::XmlInstanceSchemaManager(pModelRef);
schemaManager.ReadSchemas(status);//this was necessary to get them read in for the rest of the calls to work.
BXN::XmlInstanceApi apiRef( BXN::XmlInstanceApi::CreateApi(instStatus,schemaManager,_instanceCache));
return apiRef;
HTH,