In Timesheet Line the SUM of Quantity is not showing while Grouped by User etc. etc... want solution

<tree editable="bottom" string="Timesheet Lines"> <field name="date" on_change="on_change_date(date)"/> <field name="user_id" on_change="on_change_user_id(user_id)" required="1"/> <field name="journal_id" invisible="1"/> <field name="name"/> <field domain="[('type','=','normal')]" name="account_id"/> <field name="product_id" on_change="on_change_unit_amount(product_id, unit_amount, False, product_uom_id,journal_id)" required="1" domain="[('type','=','service')]" invisible="1"/> <field name="unit_amount" on_change="on_change_unit_amount(product_id, unit_amount, False, product_uom_id,journal_id)" sum="Total time" widget="float_time"/> <field name="product_uom_id" on_change="on_change_unit_amount(product_id, unit_amount, False, product_uom_id,journal_id)" invisible="1"/> <field name="amount" sum="Total cost" invisible="1"/> <field name="general_account_id" invisible="1"/> </tree>