If you're using a custom theme that has trouble displaying the Download Now button on the product page, try adding the following custom code to your site:

add_filter( 'somdn_product_page_content_woo', 'somdn_product_page_content_woo_custom' );
function somdn_product_page_content_woo_custom() {
	// Return a different WooCommerce product page hook
	return 'woocommerce_simple_add_to_cart';

}

Note: It's recommended that you add custom code using a plugin like Code Snippets.