rds-tmc fine on 720 fails on 520

I have just bought a refurb 520 unit. If I dock it in an existing brodit dock with an rds-tmc aerial (which my wife uses with her 720) then all I get is "looking for traffic signal". If my wife docks her 720 she gets "Updating traffic information" instantly. They are both running 8.351 - any ideas as to why is the 520 not behaving like the 720?

tomtom support talked me through reinstalling the maps and that solved the problem!