update.php blank dan data tidak mau diupdate

ini code simpannya

<?php
include "koneksi.php";

$id_guru = $_POST['id_guru'];
$id_mapel = $_POST['id_mapel'];
$id_kelas = $_POST['id_kelas'];
$jen_nilainrs = $_POST['jen_nilainrs'];
$thn_nrs = $_POST['thn_nrs'];

    $qry_tampil = mysqli_query($conn, "SELECT * FROM ruangan
      INNER JOIN siswa ON ruangan.nis=siswa.nis
      INNER JOIN kelas ON ruangan.id_kelas=kelas.id_kelas
      where ruangan.id_kelas='$id_kelas' ORDER BY ruangan.nis asc") or die (mysqli_error($conn));
    while ($data_tampil = mysqli_fetch_array($qry_tampil)){
        $nis = $data_tampil['nis'];
        $nilai_var = "nilai_".$nis;
        $nilai = $_POST[$nilai_var];
        $simpan = mysqli_query($conn, "insert into nilai_nrs values('','$id_guru','$id_mapel', '$nis','$id_kelas','$nilai', '$jen_nilainrs', '$thn_nrs')");

        if ($simpan){
            header("location:daftar_nrs.php");
        }
    }

?>

ini code updatenya

<?php
include "koneksi.php";

$id_guru = $_POST['id_guru'];
$id_mapel = $_POST['id_mapel'];
$id_kelas = $_POST['id_kelas'];

    $qry_tampil = mysqli_query($conn, "SELECT * FROM nilai_nrs
      INNER JOIN siswa ON nilai_nrs.nis=siswa.nis ORDER BY nilai_nrs.nis asc") or die (mysqli_error($conn));
    while ($data_tampil = mysqli_fetch_array($qry_tampil)){
        $nis = $data_tampil['nis'];
        $nilai_var = "nilai_".$nis;
        $nilai = $_POST[$nilai_var];
        $jen_nilainrs_var = "jen_nilainrs_".$nis;
        $jen_nilainrs = $_POST[$jen_nilainrs_var];
        $thn_nrs_var = "thn_nrs_".$nis;
        $thn_nrs = $_POST[$thn_nrs_var];

        $simpan = mysqli_query($conn, "UPDATE nilai_nrs set nilai='$nilai[]', jen_nilainrs='$jen_nilainrs_var', thn_nrs='$thn_nrs_var' WHERE nis=$nis' and id_mapel='$id_mapel' and id_kelas='$id_kelas' and id_guru='$id_guru'");

        if ($simpan){
            header("location:daftar_nrs.php");
        }
    }

?>

adakah yang bisa bantu gimana script update yang benar ? soalny dicoba berkali-kali blm nemu jawabannya