I have experimented more and realized that I cannot put a functional merge field from the related table (mentioned in the previous post) anywhere on the layout. Furthermore, there are some relationships that are allowing merge fields to be created and others that are not. And I can't find the common factor that would cause this; it seems random. Can anyone explain what rules or concepts I am missing?
I found the issue: While creating some of the tables in the relationship viewer, I used colons, " : ", as separators rather than something like a dash or underscore. By removing the colon in the title of the table, I am now able to create the merge field from the related table.