Symptoms
The Next Billing Date for a customer's subscription is in the past.
Cause
Most likely the customer account to whom subscripiton belongs was put on credit hold. In such case all operations of account's subscriptions are put on hold too and Service Terms are not being applied to them. This may result in incorrect Next Billing Date.
Resolution
As soon as subscription is released from credit hold, during the nearest Daily Billing Process it will be billed according to Service Terms.