The code has built-in debug information reporting capabilities - the trick is in enabling that reporting, and tracking down where it is sent. The easiest way to get the debugging information is to start the server from the command line using:
proftpd -nd5
This will display lots of information on the connected terminal's screen, both as the server starts up and during the servicing of any clients. If clients are having trouble logging in or authenticating, the debug messages reported by the server when a client connects are much more useful than knowing the messages displayed by the client, as the client does not know why it cannot log in. If asked to send debugging information to the mailing list, you can send the relevant snippets (if you know what the relevant debug messages are), or you can capture the debug output to a file:
proftpd -nd5 2>&1 >& /path/to/debug/file
and send that file, compressed, along with your post.
The above method works if you have ServerType standalone in your configuration file. If you run the server in inetd mode instead, and are unable or unwilling to make the changes necessary to run the server in standalone mode for the sake of debugging, then use of the SystemLog configuration directive is necessary for capturing the debug information. Add this directive to your configuration file, and add -d5 to your /etc/inetd.conf's ftp line, or to the server_args tag in your xinetd configuration file for the server. Be sure to restart inetd/xinetd so that your configuration changes will take effect.
Note that use of the SystemLog directive is not necessarily confined to inetd mode servers. If you are interested in letting your standalone server run unattended and want to have that debugging information in the log file, use SystemLog and add -d5 (or whatever your preferred debug level is) to the server startup script.
Once you have the debug output and various other information, and are still in need of help, search the FAQ, Userguide, and mailing list archives for material related to the problem. If you're unable to find anything helpful in these sources, post your question to the appropriate mailing list. Be sure to include the version used, your proftpd.conf, and possibly any debug information.
The following document describes how to ask good questions that are likely to be answered:
http://www.tuxedo.org/~esr/faqs/smart-questions.html