PDT not working in Sandbox

[snow10time]

Despite the following settings in Payment Preferences

Auto Return to ON

specifying return url in cart upload ( which overrides the necessary setting  of a url in Payment Preferences)

PDT to ON

clicking return to merchant site having made payment  opens the specified return url but does not pass any data including the tx variable

the IPN is received as normal

The same settings are also failing in a live environment and both used to work fine.

Any Ideas?