When you view campaigns or templates in Preview Mode, you may notice that your merge tags don't display as expected. Merge tags won't display live info at all inside the Template Builder, but non-list-specific tags, like video merge tags, should display when you view your campaigns in Preview Mode.
If list-specific tags display as merge tags and not live list information, it may be because the list you chose in the To section of the Campaign Builder does not have any data related to the merge tags you've used in your campaign.
For example, if you wanted to show a recipient's first name in your campaign and you used the *|FNAME|* tag in your campaign, but no one in your list or segment has data in the FNAME field, there would be no live merge tag data to show. *|FNAME|* and other subscriber-specific tags may display placeholder text in Preview Mode when there is data available in your list.
To make sure there is data in the field you are using in your campaign, either view your list or a subscriber's profile page. It's also a good idea to provide a default merge tag value, which will be used if a subscriber doesn't have data for a particular field.