Are you using Pardot Progressive Profiling?
Pardot Progressive Profiling allows marketers to progressively gather more information over time to build an insightful profile of a prospect or customer.
By default, Pardot forms display only the prospect fields that a prospect hasn’t completed yet. The Email field is always displayed on forms. For example, if your form has a First Name, Last Name and Email field, but the prospect has already completed a form with Email, First Name, and Last Name, only the Email field is displayed in the future.
Pardot gives you the ability to conditionally display a form field based on previous field completion on other forms. This allows you to show fewer fields on each form to help drive a higher conversion rate. Progressive Profiling with conditional fields enables you to capture additional information over subsequent visits.