Why would one or more of my heroes appear not appear in the town hall for stationing?

Flame Mace is a combat hero. The town hall is only to let you station development heroes in the production based buildings. (mints, rss production, training buildings).

Combat heroes are added to the formations in your legions. You need to unlock slots in the legions and then place the heroes by going to each training ground building for the legions that you have or via the legion menu. You can have up to 3 heroes in each legion but you need to complete the research for each legion to let you add them.