FTP problems

We had setup a separate site, to provide a platform for an
energy company to ftp daily statistics to our energy portfolio
managers. It was working fine til we had a ip change that went
along with an equipment upgrade of our VPS. Now their scripts
no longer work, even though they can log into the site with
an ftp client or a browser, but their scripts that automate the
process fail. They are asking about Active and Passive mode.
How do I see if this Mode setting may be the problem.
The problem comes when the script asks for httpdocs