Paypal payments go through but they don't always end in a issuing ticket

This happens sometimes. And sometimes it just works fine. I can't tell what it is that causes the following problem: I have an event with Paypal payments. When the person pays with Paypal, Zoho Backstage usually issues the e-ticket and sends it to the customer, but sometimes it just doesn't and files the payment as "failed" but the money did go through.

When this happens, I go to Zoho Backstage and go to "Failed orders" and look for the order that should have ended with an issued e-ticket. I manually change it to "offline sale" and then I resend the newly created ticket to my customer.

Most of the time it works fine. But it is not rare when the problem happens. The customer usually sends me an email telling me that Paypal charged his credit card but didn't get the e-ticket from us. I go to Paypal and I do find the payment but when I go to Zoho Backstage I notice that it didn't issue the ticket and considered it a "failed order".

Why does this happen and how can I prevent it in the future?