dbDelta not adding additional columns in plugin database update

After trial and error, here is the code that works:

function installer(){
    include('installer.php');
}
register_activation_hook( __file__, 'installer' ); //executes installer php when installing plugin to create new database

//database update checkdate
function myplugin_update_db_check() {
    global $xenon_result_db_version;
    if ( get_option( 'xenon_result_db_version' ) != $xenon_result_db_version ) {
        installer();
    }
}

add_action( 'plugins_loaded', 'myplugin_update_db_check' );

The installer.php looks somewhat like this:

<?php
    global $wpdb;
    $table_name = $wpdb->prefix . "xenon_result";
    $xenon_result_db_version = '1.2';
    $charset_collate = $wpdb->get_charset_collate();
    if ( $wpdb->get_var( "SHOW TABLES LIKE '{$table_name}'" ) != $table_name ) {
        $sql = "CREATE TABLE $table_name (
        `id` int(11) NOT NULL AUTO_INCREMENT,
        `roll_number` int(11) NOT NULL,
        `student_name` text NOT NULL,
        `father_name` text NOT NULL,
        `mobile` varchar(55) NOT NULL,
        `obj_marks` int(9) NOT NULL,
        `sub_marks` int(9) NOT NULL,
        `total_marks` int(9) NOT NULL,
        `result` text NOT NULL,
        PRIMARY KEY  (`id`)
        )    $charset_collate;";
        require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
        dbDelta( $sql );
        add_option( "xenon_result_db_version", $xenon_result_db_version );
}
if ( get_option( 'xenon_result_db_version' ) != $xenon_result_db_version ){
$sql = "CREATE TABLE $table_name (
        `id` int(11) NOT NULL AUTO_INCREMENT,
        `roll_number` int(11) NOT NULL,
        `student_name` text NOT NULL,
        `father_name` text NOT NULL,
        `mobile` varchar(55) NOT NULL,
        `obj_marks` int(9) NOT NULL,
        `sub_marks` int(9) NOT NULL,
        `total_marks` int(9) NOT NULL,
        `result` text NOT NULL,
        PRIMARY KEY  (`id`)
        )    $charset_collate;";
        require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
        dbDelta( $sql );
        update_option( "xenon_result_db_version", $xenon_result_db_version );
}
?>