There are instances when you can't find invoice numbers in QBO. See the following scenarios:
Transaction has been deleted or the number is never used.
The number of the entry was modified or edited.
Custom numbers is enabled on the company file and a user entered another number skipping the current sequence.
If you're unable to find it in the mobile app, try accessing your account in the web and see if you can find it there. To locate it in the desktop, use the magnifying glass from the top menu, and then enter the number there. Once done hit Enter to reveal the related entries.
If you can find it in the web, try to log out from your mobile and sign in again. If it doesn't work, you may uninstall and reinstall the application to remove corrupted files.
Additionally, to avoid altering the order of sales forms, make sure the Custom transaction numbers is turned off. This will help avoid unnecessary changes to the numbering sequence in your account. You can see this when you click the Gear icon and Account and Settings. On the Sales section, verify the preference and see if it's disabled.
For other concerns, feel free to post here again. I'll be glad to help you with your transactions or other tasks in QBO. Have a nice day!