I'm here to provide some troubleshooting steps so you can see those missing customers on your list. To start, let's check the inactive list on the Customers page.
In QuickBooks, click the Sales menu.
Tick the Gear icon.
Check the box beside Include inactive.
Then, search the name of the customer in the search bar.
If this works, select the Make active button under ACTION.
If you still can't find the specific customer, let's run your QuickBooks Online account using a private browser. Private mode prevents storing cache and cookies that might cause problems when running websites.
Here are the shortcut keys you can use:
Google Chrome: press Ctrl + Shift + N
Mozilla Firefox and Microsoft Edge: press Ctrl + Shift + P
Safari: press Command + Shift + N
Once signed in, go to the Sales menu and click Customers. Then, look for the missing customer.
If this works, go back to your regular browser and clear the cache. This will remove the stored cache or cookies to refresh your browser. If the same thing happens, use another supported browser. It could be that the one you're currently using has a temporary issue with QuickBooks.