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 FileCheck 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