Make sure that the Tables have Schema, Database, Owner associated with the Tables. You can associate them by Right Clicking the Table and. in Table properties, either picking up the available Schema (or in case of Oracle, even typing those directly). There you will be able to add the schema to the Table(s). See figure 1 below.
Figure 1 Schemas in the table properties editor
Finally, in the Schema Generation Options you will need to turn on Schema, Comments, and Quotes in the Other Options. See figure 2 below. Then the Schema, Comments, Quotes as selected there will appear in the Forward Engineered DDL.
Figure 2. Check on the options in the Option Selection