Most likely you are using a theme that does not support Gtk3, or has special rules for Gtk2 panel plugins but no such rules for Gtk3 plugins. At the moment, xfce4-indicator-plugin is the only Gtk3 plugin for Xfce panel - it has to be because Unity indicators no longer support Gtk2.
Try Adwaita or a new version (git) of Greybird or Numix and let me know if the problem persists.
Most likely you are using a theme that does not support Gtk3, or has special rules for Gtk2 panel plugins but no such rules for Gtk3 plugins. At the moment, xfce4-indicator -plugin is the only Gtk3 plugin for Xfce panel - it has to be because Unity indicators no longer support Gtk2.
Try Adwaita or a new version (git) of Greybird or Numix and let me know if the problem persists.