Hello, did you specify the inhert_id ?, try this:

<openerp>
    <data>
        <record id = "your_record_id" model = "ir.ui.view">
            <field name = "name">your.record.id</field>
            <field name = "model">model.model</field>
            <field name = "inherit_id" ref = "crm.crm_case_form_view_leads"/>
            <field name = "arch" type = "xml">
                <xpath expr = "/form/sheet/group/group/field[@name = 'partner_name']" position = "after">...

I hope it could help you..