I think domestically it's doing just fine. Also there's a thing called (PTA=Per Theater Average) in which "Birth of a Nation" came in 4th in the Top 10 behind 3 of those Blockbuster movies. So it's making more money per theater which is where the real box office game is. It has not open internationally. Again this is an Indy film and it needs to be judged on Indy standards. The Media is trying to spin it's numbers as if it was suppose to be this huge Blockbuster. Plus the movie is a peried piece and "R-rated", it's not G or PG or PG13 so the audience would be limited already.
What helps "Birth of a Nation" most is Nate spent the least amount of money to make the movie, it has the lower budget than many of the other slave flims (Amistad, 12 Years, Django et). The movie started in the green anyway as Fox paid double for the rights and it made History at Sundance. This is why there's is such a media push to call it a Box Office Failure because it blew out the gate already a historic success. The agenda is clear because Hollywood is upset Nate made the movie outside of the Hollywood studio system.
Also Nate broke the #1 Rule of Hollywood Slave movies, The White Savior Complex (He said while trying to get the funding some studios told him to put a White Savior in the film). I was mad at him going to Fox but I understand his stradegy now. It was about getting this epic story on as many screens as possible. There will be other revenue streams for the film (Streaming, Home video, DVD, Television etc) in the future. 21 Million Domestic will give the film 3x's it budget.