If you have notification grouping enabled for the Lock Screen, you may on occasions find that a group is not expanded when you double-tap on it

1. Ensure that VoiceOver is enabled
2. Ensure that you have notification grouping enabled for the Lock Screen
3. Ensure that you have multiple unread notifications from the same app so that they will appear as grouped on the Lock Screen
4. Navigate to the Lock screen
5. Place VoiceOver focus on to the group grouped notifications
6. Perform a 1-finger double-tap

You would expect for the group to be expanded so that all notifications in the group are revealed.

On occasions you will find that this does not happen.

This most typically happens when there are multiple notifications and groups of notifications on the Lock Screen.