When running an acknowledgment process you may run into a issue where all constituents on the revenue record are not acknowledged for a particular gift when the process completes.
With the formatted mail output each constituent must have a primary address on file and must not be deceased or inactive unless you unmark those two options within the acknowledgment process.
Steps to Duplicate
1. Add revenue to constituent record with more than one recognition credit assigned 2. Add revenue letter to revenue record 3. Run acknowledgment process (formatted mail output) against that record and that revenue letter.