YooHoo wrote:sometimes with psybnc, you need to precede commands with the word quote, as in /quote playprivatelog
That is actually with some clients, and would apply to any "non-supported" command in that client (not only psybnc-extensions). The command (if available) to send instructions directly to the server may vary, but /quote is quite common among these clients (such as IrcII and derivatives, etc).
However, since all other psybnc-commands apparently work, this is not the issue.
Also worth mentioning, if there is nothing logged, the PLAYPRIVATELOG command will produce no response at all, so it would seem there was nothing logged in this case.