Because of how cPanel works, it only allows 8 character FTP usernames, your actual username might be wrong.
Use the the truncated username.
Example: 'test12345678' is your member ID, your FTP username would be 'test1234'
Your FTP info can be verified by in Member Zone (
https://member.doteasy.com), under the heading 'Quick References', by clicking on 'FTP Info'.