Why Custom Fields are not on a Preview
When creating a campaign it is very important to do a lot of testing beforehand. One of the ways you can test is by sending a preview. When sending a preview you may notice that all custom fields along with the unsubscribe link and web version link all do not appear correctly.
The Reason for this is that you can send a preview to any email address. It does not have to be in a contact list etc. This means that there is no data attached to the email address you are sending the preview to.
If you want to make sure the fields are displaying correctly, you will need to send a live test to a list of a few test email addresses and then your fields will display correctly.