Is FB receiving the same mix as the PA in the room? If so, it is possible that you are not in the PA becuase your amp is loud enough. Is it a separate mix for FB? It is possible that you have inexperienced techs who are doing a room mix and doing a FB mix on headphones in the same room. If the bass is loud enough in the room then it will be rolling into the headphones from the room and they would think "I can hear the bass" and not add any more.
Amongst many other possibilities!