I installed qpsmtpd and ran it for a few days testing with a domain that got little or no mail traffic. I got all my plugins set up like I wanted and thought everything was good. However now that I've got a production domain set up with a pretty high volume of traffic, the clamav and spamassassin plugins are not being called for some reason.
There's nothing we can tell from the config/plugins file itself. My advice is to turn the logging level up to 11 ;-) and see what is going on. Extract the log lines for two or three connections and post those, as well as what version of Perl you are using, how you are running qpsmtpd (specifically forkserver, tcpserver, whether you are running with pperl).
John