Session variables get lost when redirecting -PHP -

i new using php , trying build content management site.

i started developing login pages , @ start session variables kept normal , working without issues when using redirect page after login confirmed.

but 2 days ago tried develop forgot password functionality , since , changing output buffering enabled have issue. session variables not carried on redirected page , login doesn't work.

my code below:

<?php include("../includes/session.php"); ?> <?php require_once("../includes/db_connection.php"); ?> <?php require_once("../includes/functions.php"); ?> <?php require_once("../includes/validation_functions.php"); ?> <?php $username = ""; if (isset($_post['submit'])) {    // process form   // validations   $required_fields = array("username", "password");   validate_presences($required_fields);       if (empty($errors)) {         // attempt login             $username = $_post["username"];             $password = $_post["password"];             $found_admin = attempt_login($username, $password);             //print_r($found_admin);                 if ($found_admin) {                   // success                         // mark user logged in                         $_session["admin_id"] = $found_admin["usr_serno"];                           $_session["username"] = $found_admin["username"];                         redirect_to("admin.php");                 } else {                   // failure                   $_session["message"] = "username/password not found.";                 }         } }else if(isset($_post['forgot'])) {      redirect_to("forgotpassword.php");   }  ?> <?php $layout_context = "admin"; ?> <?php include("../includes/layouts/header.php"); ?> <div id="main">   <div id="navigation">     &nbsp;   </div>   <div id="page">     <?php echo message(); ?>     <?php echo form_errors($errors); ?>      <h2>login</h2>     <form action="login.php" method="post">       <p>username:         <input type="text" name="username" value="<?php echo htmlentities($username); ?>" />       </p>       <p>password:         <input type="password" name="password" value="" />       </p>       <input type="submit" name="submit" value="submit" />       <input type="submit" name="forgot" value="forgot password" />     </form>    </div> </div>  <?php include("../includes/layouts/footer.php"); ?> 

the session.php starts session , admin.php check if session[admin_id] isset show page or else redirect again login.php page, happens.

any advise or please?