Here are some things to double-check:

• Make sure the CSV file has the header row and that all column headings are spelled correctly. 
  View Sample File

• Check that the First Name and Email Address fields in each row are filled in.
  
  

• Check that you don't have any invalid email addresses.  Only one email address with no extra punctuation after the domain is allowed per each row.
  
  

• Here are examples of invalid email address formats:
  
  sally@hotmail.com?
  sally@hotmail.com,
  sally@hotmail.com <Sally Jones>
  sally@hotmail.com,sandy@yahoo.com