ProFTPD 530 Error Login Incorrect

On our Debian VPS servers we run ProFTPD in order to enable client FTP access. These servers are all managed via SSH, and so very occasionally when setting up a new account I run into a 530 error: Login incorrect. You just can't get past it though, no matter how many variants you try on the password you thought you entered.

The most obvious thing to check is the password. Is it correct?