@Cristian.Soler There are a few possibilities depending on the field types, first thing to check for any field that was not filled out is to ensure the field was set to “Required”. If this was an envelope generated from a Template or Powerform you would need to edit the Template and verify the field properties to see if this was set to “Required”. For some other field types such as Checkboxes, you need to review the Validation properties of the field to ensure you set what is expected of that set of checkboxes.
On rare occasions I have seen some fields placed over the top of other fields which can cause issues but the likeliest s the “required” value is not set.