1. Make sure you've enabled Notification Mirror service on your Android device:
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2. Make sure your Android device is running 4.4+.
Go to Settings on your phone > About phone > Android version
3. If your Android device is running 4.4.2/4.4.3/4.4.4, when you get a notification pop-up includes more than 2 messages, you can't reply to this meesage. Please read those messages on your device first.
e.g. There are 2 messages in the first pop-up, and you can't reply to this message (This issue doesn't happen to devices running Android 5.0+):

Detailed guide on how to Reply WhatsApp Messages.
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2. Make sure your Android device is running 4.4+.
Go to Settings on your phone > About phone > Android version
3. If your Android device is running 4.4.2/4.4.3/4.4.4, when you get a notification pop-up includes more than 2 messages, you can't reply to this meesage. Please read those messages on your device first.
e.g. There are 2 messages in the first pop-up, and you can't reply to this message (This issue doesn't happen to devices running Android 5.0+):

Detailed guide on how to Reply WhatsApp Messages.