I had a bug report from a Debian user (which I can confirm). Basically when xfce4-panel is started, the genmon plugin is not refreshed until one visits the properties. Then, it works fine until the panel is restarted.
Downstream bug is at http://bugs.debian.org/623697 and if you want more information you can mail directly the submitter (adding firstname.lastname@example.org to CC:).
I have a similar gug report in Fedora, but I cannot confirm it:
xfce4-genmon-plugin don't show. If I right click, and select properties - don't
change any property -, close configuration window, genmon plugin is show.
If I open genmon properties window, and close, genmon is show, and I click
places applet icon, the places window show incorrect coordinates. Please see my
More info and screenshots at https://bugzilla.redhat.com/show_bug.cgi?id=695025
Created attachment 3685
Initialize display timer at startup
FYI, from having a brief look at things it would seem that this commit might have broken things: http://git.xfce.org/panel-plugins/xfce4-genmon-plugin/commit/?id=5d9618659709b820c73b547b335a8c0c160264ca
Basically it removes the initial call to SetTimer when the plugin is first initialized and replaces it with a call that is only ever run when the configuration is updated.
The attached (highly trivial) patch would seem to fix this.
I can confirm that the patch from Stefan solves the problem for me, too.
Duplicate of https://bugzilla.xfce.org/show_bug.cgi?id=7449.
*** This bug has been marked as a duplicate of bug 7449 ***