"¿Por qué no habla con su madre?"
The question doesn't indicate whether it's male or female, so singular they or he should be correct.
It would need to be "hablan" for "they" to be an option. Él/ella/usted are the options. So there are at least two other possible translations "Why does he not speak with his mother?" and "Why do you not speak with your mother?" Looking up "singular they" https://en.wikipedia.org/wiki/Singular_they it would seem to be possible to use "they" here. That is if "singular their" also exists. From what I read they are both common in speech but should be avoided in formal writing.
the sentence doesn't indicate male or female. the translation "why do you not speak with your mother" is also correct because it could be using "usted" the formal "you"
I wrote "Why not speak with your mother?" and it was marked wrong.... did I miss something?
it's not incorrect grammar, it just isn't the correct translation. Your translation conveys a different meaning.
how is "why is she not speaking with her mother?" wrong? please explain.. because thats definitely present tense..
"speaking' is continuous present though and Spanish has continuous verbs too(hablando).