This might be because Customer details or Delivery Methods have not been added to the order.
If a Customer has been added but not Delivery Method, then you can still use the Print As option on the order, and choose the correct Delivery Method. You can also send the Customer their tickets by clicking Email Order on the Order page.
However, this may also be because the email has been sent to the Customer's spam inbox, or they have unsubscribed from Line-Up emails. Contact us here if this is the case.
If a customer hasn't received their tickets, despite the email being sent out, then you may want to generate a new Barcode and resend them the ticket. Doing this ensures that the existing barcode that was originally sent out can't be used, preventing any potential fraudulent use.