Let me provide some troubleshooting steps to help you fix your QuickBooks Online app.
Too much cache on any application, such as QuickBooks Online, can cause malfunctions inside the app. That's why you can't see your invoice. We'll have to reset the app to ensure your QBO app works properly.
For iOS:
Go to your phone's settings.
Select General, then choose iPhone Storage.
Tap QuickBooks Online from the application list.
Select Offload App to free up the storage without deleting the documents and data for the app.
For Android:
Go to your phone's settings,
Choose Apps (varies by device).
Select QuickBooks Online from the application list.
Go to the Storage section, then tap Clear Data.
If the issue persists, I recommend uninstalling and reinstalling the app. Alternatively, you can use the mobile browser to view your customers list.