Looks like the Buffers menu will only show the 10 latest viewed
buffers by default. The number "10" here can be configured to fit your
need I think, but I don't know how :-( And I don't see the need to
show all opening buffers in the menu (I'm opening 600+ buffers now,
what a mess!).
Just in case you don't know it: I suggest you use the ibuffer (http://
www.emacswiki.org/emacs/IbufferMode) to navigate your buffers, it
would be more efficient than click the mouse :-)
See also buff-menu+.el. I haven't tried to track changes in CVS Emacs, but it
works fine in the Emacs 23 release (23.1.1), as well as older versions.