Q:

Wordpress - Programmatically adding products not generating thumbnails

I'm creating a custom CSV importer for a client and the pictures are added, however the thumbnails aren't being generated properly. After using a plugin like Regenerate Thumbnails they do show correctly. 
Here is the code in which I add the attachment and link it to the post.
    $uploadDir = 'wp-content/uploads/importedproductimages/';
    $siteurl = get_option('siteurl');
    $thumbnail = 'importedproductimages/' . $name;
    $filename = 'importedproductimages/' . $name;
    $wp_filetype = wp_check_filetype($filename, null);
    $attachment = array(
                'post_author' => 1, 
                'post_date' => current_time('mysql'),
                'post_date_gmt' => current_time('mysql'),
            'post_mime_type' => $wp_filetype['type'],
            'post_title' => $filename,
                'comment_status' => 'closed',
                'ping_status' => 'closed',
            'post_content' => '',
            'post_status' => 'inherit',
                'post_modified' => current_time('mysql'),
                'post_modified_gmt' => current_time('mysql'),
                'post_parent' => $post_id,
                'post_type' => 'attachment',
                'guid' => $siteurl.'/'.$uploadDir.$name
    );

    $attach_id = wp_insert_attachment( $attachment, $filename, $post_id );
    $attach_data = wp_generate_attachment_metadata( $attach_id, $thumbnail );
    wp_update_attachment_metadata( $attach_id, $attach_data );

    // add featured image to post
    add_post_meta($post_id, '_thumbnail_id', $attach_id);

Why aren't the thumbnails being generated properly?
Thank you in advance.
EDIT: 
I have also included image.php like so:
require_once(ABSPATH . 'wp-admin/includes/image.php');

A:

This ended up working for me:
            function createnewproduct($product)
        {

            $new_post = array(
                'post_title' => $product['Product'],
                'post_content' => $product['Long_description'],
                'post_status' => 'publish',
                'post_type' => 'product'
            );

            $skuu = $product['SKU'];
            $post_id = wp_insert_post($new_post);
            update_post_meta($post_id, '_sku', $skuu );
            update_post_meta( $post_id, '_regular_price', $product['ourPrice'] );
            update_post_meta( $post_id, '_manage_stock', true );
            update_post_meta( $post_id, '_stock', $product['Qty'] );
            update_post_meta( $post_id, '_weight', $product['Weight'] );
            if (((int)$product['Qty']) > 0) {
                update_post_meta( $post_id, '_stock_status', 'instock');
            }

            $dir = dirname(__FILE__);
            $imageFolder = $dir.'/../import/';
            $imageFile   = $product['ID'].'.jpg';
            $imageFull = $imageFolder.$imageFile;

            // only need these if performing outside of admin environment
            require_once(ABSPATH . 'wp-admin/includes/media.php');
            require_once(ABSPATH . 'wp-admin/includes/file.php');
            require_once(ABSPATH . 'wp-admin/includes/image.php');

            // example image
            $image = 'http://localhost/wordpress/wp-content/import/'.$product['ID'].'.jpg';

            // magic sideload image returns an HTML image, not an ID
            $media = media_sideload_image($image, $post_id);

            // therefore we must find it so we can set it as featured ID
            if(!empty($media) && !is_wp_error($media)){
                $args = array(
                    'post_type' => 'attachment',
                    'posts_per_page' => -1,
                    'post_status' => 'any',
                    'post_parent' => $post_id
                );

                // reference new image to set as featured
                $attachments = get_posts($args);

                if(isset($attachments) && is_array($attachments)){
                    foreach($attachments as $attachment){
                        // grab source of full size images (so no 300x150 nonsense in path)
                        $image = wp_get_attachment_image_src($attachment->ID, 'full');
                        // determine if in the $media image we created, the string of the URL exists
                        if(strpos($media, $image[0]) !== false){
                            // if so, we found our image. set it as thumbnail
                            set_post_thumbnail($post_id, $attachment->ID);
                            // only want one image
                            break;
                        }
                    }
                }
            }
        }

