How do I troubleshoot issue with mapping to merge fields??

Mapping fields is possible in several places in the integration. Below are notes to help you troubleshoot the most common mistakes.

• All mappings must match in data type. For example, Salesforce picklists can map to Mailchimp single-select lists or text fields. Please refer to the FAQs for checkboxes and multi-select picklists. 
• We recommend creating new merge fields from the app, with the New Merge Field button above each mapping. We will handle the data type match for you.

Data Wizard (guide)

• The Salesforce fields for mapping must be available on the wizard's source. They should be columns in the list view or report, campaign member fields, or SELECT parameters in your SOQL query.
  
  Example - To map a contact's First and Last Name instead of Full Name, you should include First and Last Name fields as columns in your report or list view.
• All Salesforce fields must be available to the running user via field-level security. This is already true if you are the running user and can include them in a report/list.

Field Mapping (guide)

• All Salesforce fields must be available via field-level security to the Salesforce Sync user. More on User Security