You will be notified if the payment fails due to insufficient funds, for example. You can view the failed payment on the Pay window under Ready for Payment.
To redo the payment:
Tap the Ready for Payment tab.
Tap on the failed bill in question
At the bottom of the screen check the second 'how' section of the arrow shaped stepper.
If required, select a different payment method.
Carry on and pay the bill as usual.
If the payment has successfully been set up you will then see it under Scheduled in the pay window.