Are there some people Botox for masseter will not work on?

Thank you for your question. Botox will always work on the masseter muscles. Although, there is people who Botox to the masseter who it does not work. This is because a wide facial shape may not be because of enlarged masseter muscles. If the patient has a bony wide jaw then reducing the masseter muscle will not make a difference. 

If the cause of square face shape is masseter hypertrophy, then the Botox will help. If the shape is the result of the flaring of the jaw bone, then Botox will likely not provide the results you are looking for. That being said, most cases that I have seen in my clinic, Botox helps. I recommend you see an experienced injector for an in-person consultation.