The two most common reasons that a transaction might fail is due to underlying orders being taken before you or the underlying order being cancelled before you confirm.
If the specific bet offer you placed a bet on was taken before your transaction is confirmed, the underlying bet will not execute and you'll be shown an error.
Alternatively, if the specific order you took was cancelled before your bet was confirmed, then the bet will not execute and you'll be shown an error.
What can I do to improve my chances of success?
To improve the chances that your bet will succeed, we recommend confirming the bet in your wallet as quickly as possible after pressing Bet.
When you press the Bet button, we automatically check that the transaction would succeed if it were to be mined in the next block. If we know the transaction will fail, then we will instantly display an error to you rather than letting you send the transaction using your wallet.
By waiting to confirm the transaction in your wallet, you are increasing the chances that the transaction will no longer be valid by the time your transaction is mined into the blockchain.