The list of available plugins is cached for one day (https://github.com/ros-visualization/qt_gui_core/blob/bccb8a2066116f105a9765146576bdedaf174263/qt_gui/src/qt_gui/plugin_manager.py#L61) to speed up repeated invocation of rqt. If plugins are added within that period they don't show up automatically since the cached information is used.
Passing --force-discover will invalidate the existing cache and ensures that the list of available is queried again (which takes a little bit of time, depending on the size of the ROS installation up to a few seconds).