Cannot integrate 'ckeditor_wiris' plugin into django-ckeditor in custom toolbar configuration

Question

I am using django to make a science related blog site where I am using django-ckeditor as the richtext editor. I encountered the following problem:

I am trying to integrate the 'ckeditor_wiris' external plugin in custom toolbar. Other external plugins - 'Codesnipet' and 'Symbol' are working fine but 'ckeditor_wiris' is not visible on the toolbar.

CKEDITOR_CONFIGS = {
    'default': {
        'height':'250px',
        'width':'100%',
        'tabSpaces': 4,
        'toolbar': 'Custom',
        'toolbar_Custom': [
            ['Smiley', 'CodeSnippet', 'Symbol', 'ckeditor_wiris'],
            ['Bold', 'Italic', 'Underline', 'RemoveFormat', 'Blockquote'],
            ['TextColor', 'BGColor'],
            ['Link', 'Unlink'],
            ['NumberedList', 'BulletedList'],
            ['Maximize']
       ],
        'extraPlugins': ','.join(['codesnippet',
                                  'symbol',
                                  'ckeditor_wiris'
                                  ]),
    }
}

WIRIS plugin not visible on toolbar :

Though if I use the 'full' toolbar instead of 'custom' then 'ckeditor_wiris' is visible on the toolbar. What can I do to make it visible on the toolbar in custom settings?