From what I know from experience, the unit must be a Neutral Hostile creep to give a bounty, but someone might tell you differently. You also must be sure that under the Units' properties, you must have
Stats - Bounty Awarded - Base (ex. 15)
Stats - Bounty Awarded - Number of Dice (ex. 3)
Stats - Bounty Awarded - Sides per Die (ex. 6)
filled out