This is a bug in psyBNC caused by a nick desync.
A quick example:
your nick on IRC is "Testing"
your nick in mIRC (before connecting) is "Hello"
when you connect, psyBNC is supposed to synchronise your nick to the nick in IRC, usually this will be seen as
* Your nick is now Testing
Sometimes it doesnt ... if this occurs, mIRC ignores joins/nicklists and there is your problem.
Its not mIRC