When placing Dutching bets some of the amounts may be under the minimum bet. When placing bets under the minimum, Bet Angel first has to place a bet at the minimum (£2) at odds of 1000 , add the odd amount then cancel the original £2 before adjusting the price down to the desired level.
This means even if you have set to use a total stake amount, Bet Angel will require more than this total stake in order to place the bet. If your account has insufficient funds Betfair will refuse the bet and a null bet reference will be returned. Bet Angel does not manage your account for you and simply tries to press on with placing the bets.
For example. Your account balance is £20 and there are 11 runners. You have asked for a total stake of £10 to be used and Bet Angel has calculated the required stakes. Which all add up to £10. However to place the bets, Bet Angel will first need to submit 11 bets at £2 minimum. Which is a total of £22. This exceeds your account balance and the bets will fail.