I've had similar problems at Denmark Hill and Crofton Park - initially I was told it was my fault, although I knew it wasn't, but when I complained online I was told there had been machine errors and the amounts were refunded although it took a long time (my impression was they were very busy dealing with similar complaints). One of the problems is that the station staff at these particular stations (at least the ones I have dealt with) don't know much about Oyster and can't interpret or refund. This is not their fault - the stations are understaffed and when Oyster was rolled out to rail services it should have been 'staffed' in the sense that it is at tube stations, where they can deal with Oyster queries.