Stupidity: With all the data gathered on all the girls, and having made a note about the relevance of "Birthday", Jack is completely incapable of establishing any connection between the girls... who all have the exact same birthday. After hours, he needs Miss Fisher to find it out. Who does it by simply reading the word "birthday" that he himself wrote.
Stupidity: Miss Fisher is a cunning sleuth and avid reader, and has to search for a hidden message. Yet she does not recognize one of the most common tropes (yes, back in the 20s already, with illustrious writers as Dumas and Poe using it!) in fiction, aka the "invisible ink", even with a gigantic clue about it. She is uncharacteristically dense in this occasion.
Stupidity: The plan of the bad guys to dispose of Miss Fisher, apparently, involved killing her and digging up a grave for her something like 20 yards away from the circus, in plain view and hearing distance from the various trailers. Since not everyone in the circus was a murderous maniac and certainly quite a few were capable to see a good opportunity for blackmail, it seems a bit unlikely it would have been successful.
Answer: Louisa's watch was broken during the struggle to get away from her attacker.
Bishop73