I had the same thing happen to me for late September of this year: called several times only to be told there was a "big convention" and I could only get 3 nights of our 9 night stay (at the gov't rate we were looking for). I just kept calling (once or twice per week), and lo and behold.....all my additional dates opened up at the Dolphin at the rate we wanted! They must have released unbooked convention rooms.
Moral of the story is: just keep trying!!