Some transactions missing in GA

Hi,

We use PayPal and Stripe for payment. Both redirect to a ‘Thank you’ page.
We see most transaction but a considerable number doesn’t get recorded.
I don’t think this is to do with customers closing the window before reaching the thank you page, because this is the only confirmation they see that the payment has gone through.

Any idea what could be causing this and how we can fix it?

Thank You,
SG