Symptoms
In credit terms Threshold to Send Low-Balance Notifications and Balance Shift to Resend Low-Balance Notifications are set, but the subscription was put on hold without additional notices about low balance.
Cause
The notification is not sent when the customer places payment but his amount is still lower than threshold, which is a bug with id #PBA-66406 (""Low Balance warning" notification was not sent.")
Resolution
The bug is fixed in update 6, update an installation to 6.0.6 or above.