I don't see merge tag info in my Preview Mode

When you view a campaign or template in preview mode, you may notice that your merge tags don't display as expected. This may be because the list that you selected during the campaign creation process has no information related to the tags you use in the campaign.

For example, if you want to display the recipient's name in a campaign, and you use merge tag {%Subscriber:Firstname%} in your campaign. However, none of the listed or syndicated recipients have data in the Firstname field. When you view the campaign in preview mode, the area where the merge tag is used will be blank.