"Why is he not happy?"
I put “他为什么不高兴”, which was marked wrong. I have flagged this, as I am fairly sure it is okay (better, in fact) to have the question word after the subject. (你为什么不去？他们什么时候来？你怎么去学校？) Any thoughts from others?
Another question in this set has 你为什么学习中文. It wouldn't accept 为什么你... I feel like I'm getting yanked around
I just typed it like you did (他为什么不高兴) and it was accepted. It was on the 17. January 2018; nevertheless he gave me an alternative word order in the sentence (为什么他。。。)
Duolingo seems to have corrected it. When I answer "他为什么不高兴?", Duolingo doesn't consider my answer wrong. Instead, it gives me the alternate answer
不开心 is actually more common for "not happy" than 不高兴. Not sure why it was not accepted.