The ticket numbers are generated upon a paid or completed order. If you use the Paypal Simple payment plugin, Paypal does not report back that the order has been paid for. So you need to go to the order, then in the upper right select, resend completed order email and that will generate the tickets and send them to the customer and add them in the order info on the woo commerce admin. To do this automatically, requires a payment module that reports back that the payment has been completed like Paypal Pro or Paypal advanced. There are plenty of Paypal payment plugins available.