Ping from Remote Users to FTP server

I have many remote users that connect to our FTP server through the
Internet. From time to time, it tells them they can not connect to us
because of an Internet error. I use the PING, FTP and Tracert commands
to see how they are connecting to us. I don’t know what the PING tells
us except that it is a reachable address. Plus, we get their IP address
and try to ping them back and can’t. What is that telling us? So in
essence, they can get to us but we can not get to them. Whatis that
telling us?