content page is not getting posting back after changing the master page drop down list item

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ASP.NET

master page

<asp:DropDownList ID="drpCountry" runat="server" AutoPostBack="true" OnSelectedIndexChanged="drpCountry_SelectedIndexChanged">
                            <asp:ListItem Text="India"></asp:ListItem>
                            <asp:ListItem Text="USA"></asp:ListItem>
                        </asp:DropDownList>

master.cs

protected void drpCountry_SelectedIndexChanged(object sender, EventArgs e)
   {
       Session["country"] = drpCountry.SelectedItem.Text;
   }

content page.cs
protected void dlTopProducts_ItemDataBound(object sender, DataListItemEventArgs e)
{
if (e.Item.ItemType == ListItemType.Item ||
e.Item.ItemType == ListItemType.AlternatingItem)
{
Label lbl = (Label)e.Item.FindControl("lblPriceLogo");
DropDownList drp = (DropDownList)Master.FindControl("drpCountry");

if (drp.SelectedItem.Text.Trim()== "India")
{
lbl.Text = "Rs";
}
else
{
lbl.Text = "USD";
}

}

}