First, check to make sure that there is a value for that custom field associated with the contact in your contact database. If there is no value for a particular custom field, a space will be substituted instead.
You can search for a specific email address from your Contacts page, your All Contacts page, or from within a specific list or segment. Simply type the email address, or part of the email addres, you are looking for in the search bar. Once you’ve found the address, navigate to that contact’s profile to check the value of the custom field.
If you do find that the custom field has an associated value on the contact’s profile page, check the spelling of the substitution tag in the content of your campaign.