Your ftp account may not be the same as http. Login to your site host using http and got to ftp TAB (or similar setting). There you will see a list of ftp account accounts that were created by default. You may notice that these are not the same as your http account. Only the username will be different, the password will be same for both http and ftp accounts.
Also note that you may need a different server name for ftp account as well. For example ftp.mydomain.com. I have had sites which uses http address for server name and http account for ftp access. I also have account which uses a different account for ftp with a different server name as well.