Here are a few possibilities to look at.
Insufficient bank or credit balance
Maxed out credit/charge limit for the day — you can change this through the card issuing bank
Issuing bank rejects multiple transactions from same merchant (Refer to the types of transactions that can occur with GetGo. You can inform the card issuing bank to accept future GetGo transactions.)
In most cases, we'd encourage you to let us know of this issue if it happens through our Live Chat. At the same time, do check in with your card's issuing bank.