all values not update in sql from a page

i have a grid view when click on edit then this redirect to a page where all data read but when i m upadate data abd press submit button then they are not updated
my Code:
protected void btn_Submit1_Click(object sender, EventArgs e) { if (Session["ID"] != null) { string sql; sql = "UPDATE tbl_Agents SET Title='" + drop_title.SelectedValue.ToString() + "',Forename='" + txt_forename.Text + "',Surname='" + txt_surname.Text + "',Occupation='" + txt_occupation.Text + "'"; sql = sql + " ,Mobile_Number='" + txt_mobile.Text + "',Fax_Number='" + txt_fax.Text + "',Email_Address='" + txt_email_address.Text + "'"; sql = sql + " ,DOB='" + txt_dob.Text + "',Gender='" + drop_gender.SelectedVa ...