Use fields from related objects in a Mail Merge

Related lists associate the record to multiple associated records. Because of this association to "many" records (regardless of it's actual associated record count) the mail merge will not be able to specify which record to pull values from, and will not populate any of the fields for the related object.

When working with a custom object, there needs to be a Master Detail (Parent Child) Relationship between the objects. If this relationship is set as a lookup, the values from the secondary object will not be included within the merged document.

Example:

When running a merge from a contact record, include fields from the Account record. The relationship from the Contact to the Account allows only one Account and so the merge will complete successfully.

Attempting the same merge from an Account record, results in merging to a multiple record relationship as there can be several Contacts on a single Account. This will cause the merge functionality to populate the Account data but none of the information for the Contact will display.

There is one exception to this. When attempting to use Contact details within an Opportunity mail merge,  a Contact to merge the details can be selected.