We have been using PayPal to accept payments on our site for several years. What has worked for years is that the PayPal IPN notification activates the purchase.
We recently moved hosting company and the IP has partially stopped working.
When PayPal first sends the IPN it shows the status as 200 in our PayPal account but the purchase does not activate.
But when we resend the IPN it does activate the purchase.
We feel they must be taking a different route back.