I've experienced the same problem since upgrading to 4.7.3 and beyond. What I found is the IPN is sending the wrong path to Paypal, so that Paypal cannot update.
This is a response from paypal themselves:
Verify that the server is generating automatic Paypal payment notifications (IPN). The IPN sent to this URL have failed :
Basically it is looking under components and not under administrator/components.
Here's what i am trying - I created the path/file under components.
I'm trying that now. I figure if it can't find the file because the wrong path is specified, I'll copy the file there where it wants it to be.
If I knew where to fix the error I would - does anybody know?