Proclaim Font is controlled by the Proclaim font style, we need to make sure letter and letterhead both have proclaim font selected:
Select the letter in Workflow Maintenance
On Style Option Click on small Arrow
Make sure Proclaim Font is Selected
Also Check Proclaim Font style has Right font Selected.
If you or your system administrator don't have access to Workflow Maintenance, please raise a new case online and reference the title of this article.