If an invoice isn't showing in your QuickBooks Online mobile app, I've the following suggestions for you:

Begin by refreshing the data within the app through the "Help & Feedback". If the invoice remains unseen, restart your phone.

If the problem persists, uninstall and reinstall the QuickBooks Online mobile app. Ensure a stable internet connection, allowing a few minutes for data to sync after each step.

As a final option, access and view the invoice by logging into QuickBooks Online through a mobile browser on your device.