On April 28th the Atlanta Braves were 17-7 having allowed 27 fewer runs than the next best team and were 3 1/2 games up on the, then in second place, New York Mets and 6 1/2 games up on the now 2nd place Marlins. Since that day the Braves have gone 15-21, 9-11 at home and 6-10 on the road while scoring just 118 runs and allowing 140.
The short answer is they can't hit, but that has always been the case. Even on April 28th just 3 teams had scored fewer runs than the Braves, but the Braves were allowing just 2.42 runs per game, something not even remotely sustainable for any team, let alone a team with black holes at 2B and 3B offensively and defensively.
If you check the odds at allpro the Braves have fallen to 8-1 favorites to win the NL pennant even though they're currently leading their division. On the bright side rookie Tommy LaStella has played well in place of Dan Uggla since he's been called up, but it's only 27 at bats. BJ Upton will never turn into a walk machine, but he has improved this year at the plate, enough at least to hold off Jordan Schafer for everyday at bats although Upton should really be sitting vs righties with Schafer is those days.
The Braves have played the 7th toughest schedule in baseball and have 13 games left against the dreadful Mets and Phillies, but I don't know if it will be enough to hold off 2of the Nationals, Marlins, Dodgers and Cardinals for the final playoff berth.