If proftpd is not letting you login, then there are few things you can do check to see why that is.
1) Try resetting your password to ensure it all matches up.
2) Check the /etc/proftpd.vhosts.conf to see if there are any virtualhosts for the IP you're connecting to. If there is, check the password file it points to, to ensure the account they're logging in with exists in the file. If no virtualhost means it defaults to /etc/proftpd.passwd (check the /etc/proftpd.conf to confirm)
3) For any shared IP account, the user/cryptedpass should be in /etc/proftpd.passwd Owned IPs should be in /usr/local/directadmin/data/users/username/ftp.passwd, but will have a virtualhost in the proftpd.vhosts.conf file.
3) failing that, try running proftpd in debug mode: