Session variables get lost when redirecting -PHP - php

Question

I am new using PHP and I am trying to build a content management site.
I started developing the login pages and at the start the SESSION variables were kept as normal and were working without any issues when using redirect to another page after login was confirmed.
But 2 days ago i tried to develop the forgot password functionality as well and since then and by only changing the output buffering which i just enabled I have an issue.
The SESSION variables are not carried over to the redirected page and thus the login doesn't work.
My code is 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 the 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 as 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 $layout_context = "admin"; ?>
<?php include("../includes/layouts/header.php"); ?>
<div id="main">
<div id="navigation">
</div>
<div id="page">
<?php echo message(); ?>
<?php echo form_errors($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 the session
and the admin.php will just check if the SESSION[admin_id] isset then it will show the page or else it will redirect again to login.php page, which is what happens.
Any advise or help please?