By default, if you have services that are being renewed at the same period, the renewals invoices would be combined into one. It provides a greater convenience upon payment as it prevents to repeat the renewal steps several times.
Punctually, you may need to separate an item from an invoice because you aim at cancelling a / some service(s). To proceed, you would simply need to place a cancellation request from your client area for the service you would like to cancel and within the next 24 hours past the cancellation request, the invoices would be updated accordingly.
However, if you would rather to have the invoices being separated by default, you would need to open a support ticket at the Billing Center mentioning that you would like to have us enable the automatic separation of invoices.