Hey there! I came across this thread and wanted to share an updated guide here 
This usually happens because PayPal redirects customers away from the funnel, so the Facebook Pixel doesn’t always fire on the thank-you page.
We put together a step-by-step guide on how to fix this and improve tracking accuracy: How to Fix Facebook Pixel Not Tracking PayPal Purchases
Hope this helps - feel free to reach out if you need a hand setting it up! 