When I open a folder with unloaded images, the new thumbs are not visible until i 'force' an expose event on the icon (hover icon, resize window). Meaning that when tumbler reports the thumb is generated, thunar does no call gtk_widget_queue_draw(_area) on the actual icon on screen. Running both latest master of thunar and tumbler.
Because of the wrong dbus type, the function probably did complete, because now it works fine. See commit http://git.xfce.org/xfce/thunar/commit/?id=0c5e0a03be51cef6d342ac8e1c480378ffba6487