Email-based users will only receive notification emails after having successfully registered their account.
If there are users on the Campaign Schedule Report who do not have an Email Due Date assigned, this is likely because their account is unregistered.
The example below shows an unregistered user with the User Status as Inactive.
Once the user successfully registers their account, the Email Due Date will populate.
Content with Direct Access enabled, and Direct Access for Unregistered Users selected within 'Company Edit', may show users as 'Inactive' with an Email Due Date assigned.
See Do All Users Need to Register? – MetaCompliance for more information on Direct Access.