If you're encountering a "busy" message from the UK ETA application and unable to proceed, follow these steps to potentially resolve the issue:

1. Check App Status: Verify if there are reported issues with the UK ETA app. Sometimes the service may be down or undergoing maintenance. It’s advisable to check forums or social media for user reports on the app’s status. For example, Reddit discussions often provide real-time updates from other users experiencing similar issues - learn more.

2. Restart the App: Close the app completely and restart it. This simple action can resolve temporary glitches that may be causing the busy message.

3. Clear Cache: If you're using the app on an Android or web platform, clearing the app or browser cache might help. This can remove any outdated data that might be causing interruptions.

4. Update the App: Ensure you have the latest version of the UK ETA app installed. Go to your app store (Google Play Store or Apple App Store), search for the UK ETA app, and check if an update is available.

5. Try Different Times: Since the server may be busy during peak hours, try accessing the app at different times of the day, particularly late at night or early morning.

6. Scan the Passport Correctly: Ensure that any cover on your passport and phone is removed, and try scanning from the back of the passport instead of the front. This is a common tip shared by users who faced similar scanning issues - learn more.

7. Internet Connection: Check your internet connection. A poor connection can lead to issues with communicating with the app servers.

8. Contact Support: If the issue persists, consider reaching out to the UK Visas and Immigration (UKVI) for guidance. They can provide updates regarding app functionality and any ongoing issues - learn more.

Following these steps should help you in troubleshooting the issues with the UK ETA app. If all else fails, patience may be required until the server load decreases.