Android Firebase Custom Notification not working when app in backgroud

I am encountering a problem with Firebase Custom Notifications on my Android device. The notifications are not functioning as expected when the app is running in the background.

Ideal Skills and Experience:
- Proficient in Android app development
- Experience with Firebase and custom notifications
- Knowledge of handling background processes in Android
- Ability to troubleshoot and resolve notification issues

Please provide a detailed proposal on how you plan to address this issue and any relevant experience you have.