Strange plugin issue on IE9

We have been converting our CMS plugins from V2.6.6 to 3.6.2 using the iframe method.

Plugins are working well except that we found the following strange behaviour, only in IE9.

When you open the dialog box in IE9/win7, you must click the dialog box for the content to appear.
It works fine on IE8 / Opera / FF.

This is the js to open the plugin in iframe mode:

CKEDITOR.dialog.add( 'wsimage', function( editor ) {
   return {
      title : 'Images', minWidth : 810, minHeight : 510,
      contents : [ {
            elements : [ {
                   type : 'iframe',
                   src : CKEDITOR.basePath + 'wsplugins/wsimage/dialogs/wsimage.cfm?publication_id='+publication_id+'&f='+ws_fld_name+'&story_id='+story_id,
                   width : 810, height : 500 - (CKEDITOR.env.ie ? 10 : 0)
            } ]
      } ]
      , buttons : []   // don't show the default buttons
   };
} );

Would anyone know how to solve this issue?

Thanks