Like everybody suggested, you should get the strong recommendation letter from your advisor, head of the dept., and consultation from int'l advisor. I think you'll get it eventually. Please also do not argue with them in your next interview or show your frustations and never call it visa renewal. I was reading transcripts from chat interview with ktm visa consular, and she mentioned there's nothing called "visa renewal". They said they look at it just like new application. But obviously, if you've not violated any rules in US, I guess everyone applying for visa ("renewal") must get it.

Finally, when you go next time, clearly mention the tentative data of your graduation and the work that you've finished till now (MORE) and the LESS work remaining for you to complete Ph.D.

GOOD LUCK!