They don't necessarily have to credit every report they see as accurate to recognize the news teams may be on to something and send a team to check it out. Presumably they had small teams of officials deployed throughout the area (boots on the ground) to look into problems.
Why were't they checking it out? I don't know but the excuse that the state authorities didn't tell them rings hollow to me.
I won't disagree with your concern, but it is easy to jump to conclusions of incompetence in this unprecedented situation. I am not prepared to say that all that could be done was not being done. In retrospect, could more have been done? Of course, had we known then what we know now much more would have been done. I suspect the mayor might even have used the school buses that now sit flooded not two miles from the Superdome.
I do understand that Gov. Blanco has YET to allow the national guard to be federalized. That is inexcusable.