Button not clickable in list view

Hello Community, I try to add a button in a list; the view appears, the button appears too but it is not clickable. It's like in a ghost state and I am unable to have the focus on it. Here the code : >pkg-cfg.treepkg-cfg_linesformform and the python code for the function :

   def add_line(self, cr, uid, ids, context):
        print "ENTER IN pkg-cfg_lines.add_line"
        return True

When I try to click on the button, the action is never called.... I also tried using a button with **action** type for the button : < button name="%(add_line)d" string="Add" type="action" icon="gtk-go-forward" /> with the following action : ir.actions.serverTestingTruecodeaction=obj.add_line(context=context) Any ideas ? Thanks in advance, Marc