Resolution:
When fetching schemas for a custom IDoc type or an Extension on BusinessStudio, the custom IDoc/Extension must be released by the SAP system, otherwise the schemas will not be fetched. Also, the segments which are added to the custom IDocs/Extensions are required to be released too. The same requirement does not exist for the adapter palettes in TIBCO Designer. The schemas of custom IDocs/Extensions can be fetched by adapter palettes regardless whether the IDocs/Extensions are released by SAP. There are menu actions of transaction WE30 for releasing the custom IDoc/Extension and WE31 for releasing the segment on SAP system.