system not set invoice for disabled services. As I see, you use Postpay billing, so, probable you disabled service in this month, then system will calculate only active period for that service.
If you want, I can check your customer but I have to know his ID. And better to create ticket)
Hi Alvin, please use tickets for this type of questions - https://splynx.com/my-tickets/
Our engineers can check it then in your server directly and help to resolve