We're happy to see you here in the Community, mark. Let's collaborate to produce your required report in QuickBooks Online (QBO).

Your monthly income may be inaccurate because of an incorrect report or inexact filtering of the reporting period. Let's run the Sales by Customer Summary report to check your sales per customer during a specific period.

Here's how: 

1. On the Reports tab, select Reports.
2. Search the Sales by Customer Summary report on the Find report by name ▼.
3. Fill the Report period accordingly.
4. Click Run report.

Also, if you want to check your total invoices, you can customize the Sales by Product/Service Detail report. To do so, you can follow the steps I'll be providing below:

1. On the Reports tab, select Reports.
2. Search the Sales by Product/Service Detail report on the Find report by name ▼.
3. Fill the Report period accordingly.
4. On the Group by ▼, choose Transaction Type.
5. Click Customize. Then, on the Filter ▼, select Invoices under ▢ Transaction Type.
6. Press Run report.

Note: you can switch from the Cash to the Accrual method and vice versa if your wanted transactions still don't appear on your data.

Additionally, you can access this article if you need to manage your inventory in QBO: Set up and track your inventory in QuickBooks Online.

Don't hesitate to reply to this thread if you're referring to another report or if you have additional QuickBooks-related queries. Have a great day!