Turn on email archiving and install the email archive viewer addon. If the emails show up in the archive then Zen Cart is sending the email but it's being dumped by one of the many mailservers between yours and the customers'. If they don't show up in the archive (but other emails do), then something's broken in your code.
When these emails are allegedly "not sent", do the customers see the checkout-success page with an order number and a link to their My Account area? Or, what happens after they click "confirm" on the checkout-confirmation screen?
What addons do you have installed?




