Change the actions of a button, don't work. Why?

Hi,
Still learning how all of this work, . I want to add a button in an existing addon, but i loose my hair on that. So i thought:
Why not simply use a button that is of no use to me, and replace the <onclick> value in my advantage.... but NO, Why?

For the moment, the installed script is for test, it only open a xbmcgui.Dialog().yesno(), and launch a xbmcgui.DialogProgressBG(). Allworking

the item i modify:

xml:
<item>

<visible>!String.IsEmpty(Container(10051).ListItem.Plot)</visible>

<property name="icon">plot</property>

<label>$LOCALIZE[207]</label>

<onclick>RunScript(script.embuary.info,call=textviewer,header='$ESCINFO[Container(10051).ListItem.Label]',message='$ESCINFO[Container(10051).ListItem.Plot]')</onclick>

</item>

changing only <onclick line:

xml:
<onclick>RunScript(script.tmdb.downloader)</onclick>

but it still open the  no use plot window.

Whats wrong with that ?
Thanks