Hi Gavin,
This may have been a Betfair problem but last night doing triggered back betting on every runner in the race, whenever a race had more than six runners the Status column would show FAILED and the Report column would show ERROR-IN-ORDER. I would also get the insufficient funds message shown on the bottom of the screen despite having the required funds. I had to resort to simultaneously sending 6 bets at a time rather than say 10,13,16 etc.
Thanks,
Ray
PS last night was the first time I had tried multiple bet placement.