I use procmail for this, not postfix, but I've had the same problem where my installation of SpamAssassin is not being reliably called.
I'm getting intermittent, but frequent errors like:
procmail: Executing "/home/sborg/bin/spamassassin"
/home/sborg/bin/spamassassin: /home/sborg/bin/spamassassin: Transport endpoint is not connected
My suspicion is that something in the clustering is broken, and that some hosts don't have access to my main home directory, where SA is installed.