Reach Challenges: Cook 'Em and Clean 'Em

Pretty regularly, there are daily challenges for getting jetpack kills.  These can be frustrating to get because it may seem inconsistent about whether or not you get credit for your jetpack kill.  The key here is that the enemy has to die while your jetpack is active.  That means your are holding down the jetpack button while he dies.  If you have been flying around and feathering your jetpack (like you're supposed to) and get the headshot between bursts, it will not count because the game doesn't recognize that your jetpack is on while you get the kill.  You can get kills however you want - body shots, head shots, grenades, melees all count - as long as you are holding the jetpack button when they die.  You will see the Firebird medal pop up when your kill registers correctly.

Now that you know, knowing is half the battle.