Why are the app and the website not syncing reviewed transactions?

Thank you for posting here in the Community, linahallenbeck-g. 

The QuickBooks Online Mobile App is always in sync with QuickBooks Online on the web. If may I ask which side has a missing transaction? If it's from the QuickBooks Online App, you may refresh app data. 

Here's how: 

1. Choose the QuickBooks app from the List.

2. Go to Menu ☰.

3. Select Help & Feedback. 

4. Look for Refresh Data. 

5, Tap Refresh. 

If the issue persists, I'd suggest uninstalling and reinstalling QuickBooks Online Mobile App. 

However, if you're accessing QBO on the web, you can clear your browser's cache

You can read this article to learn more about the features that you can access in the mobile app: Compare mobile app features.

Keep me posted if you have other questions about using the Quickbooks Online Mobile App. Have a great day!