That’s odd. A bill is usually shown as 'Due' when a new bill is generated or the old one is yet to be paid. If your new bill has not been generated yet, please click on the “Refresh” button against the bill on your bills & recharge page. 

If you are sure you have paid the bill, check the transaction status. It should reflect one of these: 

Pending: The status will be updated to 'Successful' as soon as we receive confirmation from the biller.

Success: If this is the case, then reach out to your operator. 

Failed: You can initiate a refund or re-initiate the entire transaction after some time.