HostBill by default tries to generate all invoices to customer services on one invoice, only if those services have matching due dates and invoice generation settings.

To make sure customer will receive one invoice for his services next month, make sure that next due date for his services matches each other:

In Orders & Accounts -> Accounts -> [Account details]

In products related to customers accounts make sure invoice generation settings match

In Settings->Products & Services -> [Product Details]