The sentence is ok, but you said "why haven't you washed yourself yet?". If you want a true translation, it has to be: Pourquoi ne vous êtes-vous pas encore lavés le visage ?

Oui, c'est correct !

Shouldn't that involve a reflexive verb? Pourquoi vous ne se lavez pas ton visage encore? I look forward to hearing from someone who has an answer, not just another question like me.

Pourquoi ne vous êtes-vous pas encore lavés ? I wanted to add a comment, about the word 'lavés' I think the "s" at the end is related to what it is washed, if it is hands, it will be "lavées" , if it is the face, in this case it will be "lavé" without "s" at the end, because it is singular and masculine