I can see that having the feature to track your customer's invoice is needed for your business. Mtjorgo.

In the general process, you'll see the Viewed option once the invoice has been opened. 

But, once they have been paid, you'll only see the Opened and Paid option. 

However, if it's the invoice activity that won't show up, we can perform some troubleshooting steps to fix the issue. Often, if you run into these problems, this is about browser-related issues. We can perform some troubleshooting steps to fix the problem. Use the shortcut keys below to access a private window (incognito):

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Safari: Command + Shift + N
• Microsoft Edge: Ctrl + Shift + P

When using incognito mode, this deletes cookies and browser history leaving your browser clean and easy to access any websites and error-free. After that, you can double-check if the option is already showing. If so, go to your main browser and clear the cache to fix the problem. Cached files and data can sometimes cause a webpage to reload. Also, some browsers have a setting that can cause a page to reload automatically. If it is still not working, you can switch to another supported browser.

On the other hand, if the status remains as opened or sent, even if the customer has already paid invoices, I recommend contacting our support team so they can double-check this and investigate further. They have the tools to pull up your account in a secure environment. 

Furthermore, learn how QuickBooks Payments allows you to add links to invoices so customers can pay online. I've added this article for more details: Take and Process Payments In QuickBooks Online and QuickBooks Payments. 

Please let me know in the comment section if you have any other concerns with invoices. I'll be around to help you. Take care always!