If a mail merge field is added to an Email Campaign template (How do I add Mail Merge fields to my email campaign?), the mailing list(s) must contain the mail merge field and the field must have data.
In Summary,
a) Actions > Create Email List can be used to create a new list, or update (add new contacts) to an existing list
b) once a contact is added to the mailing list via Actions > Create Email List, the data is static so a new list should be built, or the contact removed from the existing mailing list and re-added using Actions > Create Email List
c) all contacts should be added to the list using Actions > Create Email List in order to pull in the additional mail merge fields
d) an email campaign should use similarly created mailing lists in order to use mail merge fields, otherwise, the only mail merge fields available will be name and email