This can occur if the final payout for the bet is less than what the bonus bet stake size was. To explain in detail, see the example below:
After placing the Quinella bet below, after using a $10 bonus bet, we will get 47.61% of the Quinella dividend.
As the Quinella paid $4.60 here, and because we used a bonus bet as our stake, our payout is calculated like this: 0.4761 x 4.6 - 10 = -$7.8
So, in this case, our payout would have been a negative value (-$7.8), meaning there is no payout.