It will prompt for user pass even if the user doesn't exist. First thing to try, make sure the users can actually login to the system from the system first. It might just be a password problem with the system. Also, the error you get when you do ftp, that'll be helpful. Lastly, check your log files. With proftpd.conf you can set where you want your log files to be, or just look in /var/log/messages or your similar system log (this might be /var/log/everything or /var/log/syslog).
I personally use:
/var/log/proftpd.log but that is setup in my proftpd.conf file, with a section:
If you do make changes to this file, be sure to restart it. This brings me to my next point, are you running it as a daemon (inetd or xinetd) or standalone. This should be shown at the top of your proftpd.conf file which one you are using.