Per the documentation What is the Program board in my plan?, the board only shows issues at the level 1 and level 0 hierarchy levels.

This view focuses on issues at the level 1 and level 0 hierarchy levels (commonly referred to as the Epic and Story levels). Any hierarchy levels above or below these – such as Initiative or Subtasks – won’t show on this view.

It'd be great if users could customise the hierarchy levels to be shown on the board.