我的登录名不会重定向到PHP中的索引

Question

Hi I'm doing a login with roles for example if a user is administrator or a seller have to redirect to their repective index but when I clicked in the login button it redirect me to the same login.php page and I don't know why.

Here is my code

login.php

    <section class="login-block">
    <div class="container">
    <div class="row">
        <div class="col-md-4 login-sec">
            <h2 class="text-center">Iniciar Sesión</h2>

<form class="login-form">
<form action="process_login.php" method="POST">
  <div class="form-group">
    <label for="exampleInputEmail1" class="text-uppercase">Usuario</label>
    <input type="text" class="form-control" name="usuario" placeholder="">
  </div>

  <div class="form-group">
    <label for="exampleInputPassword1" class="text-uppercase">Contraseña</label>
    <input type="password" class="form-control" name="contraseña" placeholder="">
  </div>

  <div class="form-check">
    <label class="form-check-label">
      <input type="checkbox" class="form-check-input">
      <small>Recordarme</small>
    </label>
    <button type="submit" class="btn btn-login float-right" name="btn_entrar">Entrar</button>
  </div>
</form>
</form>

And my code php process_login.php

<?php
include 'db.php';

if(isset($_POST['btn_entrar']))
{
    $user=$_POST['usuario'];
    $password=$_POST['contraseña']; 
    $admin="Administrador";
    $vendedor="Vendedor";

    $query_admin = "SELECT * FROM empleado WHERE usuario='$user' 
              AND contraseña='$password' AND cargo='$admin'";
    $ejecutar_adnin = mysql_query($query_admin);

    $query_ven = "SELECT * FROM empleado WHERE usuario='$user' 
              AND contraseña='$password' AND cargo='$vendedor'";
    $ejecutar_vendedor = mysql_query($query_ven);

    if(mysql_num_rows($ejecutar_adnin)==1)
    {
        header("location: index.php");
    }
    else if(mysql_num_rows($ejecutar_vendedor)==1)
    {
        header("location: index_empleados.php");
    }
    else
    {
        echo "<script>alert('Usuario o contraseña son incorrectos')</script>";
    }
}