There are a few reasons why this occurs:
1. The Order was Confirmed in the Back Office and the Customer has yet to enter their Card details via the payment request email you sent to them
2. The Order has been saved but not yet Confirmed
3. The authorization for the payment has expired. After the Customer enters their Card details to authorize the payment, you have 30 days to Charge Card. After 30 days, the ability to Charge disappears and you will need to use the Reconfirm button to send a payment request email, so the Customer can once again provide their Card details.