We have noticed that with some PayPal implementations the sales tracking will not fire correctly because the user closes their browser after the PayPal confirmation screen. You can switch Autoreturn on so that the Sale can be tracked.
The link below has all the information you need. Please let us know if you tracking data does not work after this setting is on.
https://www.paypal.com/au/cgi-bin/webscr?cmd=p/mer/express_return_summary-outside
Article ID: 79, Created: 5/22/2014 at 3:04 PM, Modified: 9/4/2014 at 5:08 PM