Users of ProFTPD will often encounters problems. It happens with all software, not just ProFTPD. How, then, does the user track down the cause of the problem, and fix it? This is the art of debugging. When users post these problems are the mailing lists, it is extremely helpful to include the following bits of information to help find the answer. Even better is when the user follows these steps and determines the solution for themselves.
Various problems afflict various versions of the code, so when tracking down problems, it is good to know the version being used:
proftpd -vv
It is possible that the problem you are encountering is due to some bug that may already be fixed in a more current version, fixed in the CVS repository, or has a bug report with an attached patch. Searching http://bugs.proftpd.org will often yield useful information, depending on the keywords used in the search.