This problem in ongoing. And also affecting others:
http://discussions.flightaware.com/flightxml/getlasttrack-no-data-available-t36193.html
I would appreciate some indication of where FA is on this. Is this failure rate considered acceptable, or is there work ongoing that is trying to solve this problem? In my case all the flights reaqlly were in the air 24 hours earlier, and are all over southern England, so ADS-B coverage is good.