On Pokemon Diamond I have a female Skarmory with Defog and a Male Starly with Brave Bird. But the offsprings aren't being passed down BB.
I'm sure that starly has BB because its at lv.40 and I took him out and double checked
Does Skarmory have 4 moves? Delete one those moves and that should be replaced by BB. If the Starly had 2 or more of Skarmory's egg moves, delete them as they may replace BB.
hope I helped.