It's not a failure on the President's part, it's a failure on all of us.
Though, let's be honest, the Republicans carry a lot of the load:
The second and related reason that it is impossible for Obama to unify us after this tragedy is that it is tied up with race, and there are significant numbers of white people who will always believe that on issues of race, Obama is intentionally trying to set Americans against each other, no matter what he actually does or says. Any reasonable observer would look at his statements about racial controversies and see someone being painfully careful and tentative, struggling to confront the reality of discrimination and racism without offending whites. But for his trouble, he has been cast by his opponents as a racial avenger, some kind of Black Panther using the powers of the presidency to wreak vengeance upon innocent white people.
Republicans have told themselves a story in which the nation was moving toward racial harmony until Barack Obama came into office and immediately began dividing us over race, pitting blacks against whites and tearing the country asunder. And they have been telling their constituents this from the moment he took office. No one familiar with conservative media can deny that it has featured a festival of race-baiting since 2009, blaming Obama for every racial incident anywhere and casting all his policy decisions as motivated by the desire to stick it to white people. Some black kids beat up a white kid on a school bus? “Obama’s America, white kids getting beat up on school buses now,” Rush Limbaugh, the most popular radio host in America, tells his listeners. “You put your kids on a school bus, you expect safety but in Obama’s America the white kids now get beat up with the black kids cheering, ‘Yay, right on, right on, right on, right on.'” He passes a stimulus bill? “Obama’s entire economic program is reparations,” Limbaugh says. And that was all from his first months in office.
It's a very good piece, and you can read it at the Washington Post.