This is how it works, but if you wished restocked products to always be immediately available again, you could tweak the restock function as follows:
find the following line in the zen_restock_order function in admin/includes/functions/general.php
PHP Code:
set products_quantity = products_quantity + " . $order->fields['products_quantity'] . ", products_ordered = products_ordered - " . $order->fields['products_quantity'] . " where products_id = '" . (int)$order->fields['products_id'] . "'");
and change it to
PHP Code:
set products_status = 1, products_quantity = products_quantity + " . $order->fields['products_quantity'] . ", products_ordered = products_ordered - " . $order->fields['products_quantity'] . " where products_id = '" . (int)$order->fields['products_id'] . "'");