In the past I found that the readers at the barrier at Forest Hill seemed less sensitive than those on the Underground. I had on a few occasions to take my freedom pass out of my wallet to get it to register. Not a a problem recently though.
On another point, Deutsche Bahn is excellent news. If you find the National Rail enquiry site is working slowly, go to http://www.bahn.de which will give you the same information in an instant (and also produce timetables for such useful journeys as Herne Hill to Samarkand - great fun). The German site may not always be updated for service disruption though, which may be a consideration round here.