Why can’t I preview the contents of new messages on lock screen?

You can set whether to show senders and message contents on lock screen. Both of these settings are persona-based.

Sender Preview - to show the senders’ names on lock screen
To enable sender preview, please go to Dashboard > enter a specific Persona > Tap the pencil icon on the top left hand corner > “Notifications” > Turn on Sender Preview.

Text Message Preview - to show the contents of text messages on lock screen
To enable text message preview, please go to Dashboard > enter a specific Persona > Tap the pencil icon on the top left hand corner > “Notifications” > Turn on Text Message Preview.

On the other hand, you can also set not to show sender and text messages on lock screen.