Problem with the circus theme might be that its a one trick pony. You can't really use it for much else than circus performances.
It's beautifull, but it has a far more limited range of play possibilities compared to a house, castle or pirate ship for example.
My gues would be that the firemen theme is not a big seller either, again due to limited possibilities.
Historic themes probably depend on each batch.. Pirates are probably doing well in each batch, might be one of the most popular playmobil themes. I'd gues its outselling the medieval theme in recent years ever since medieval limits itself to just castles and soldiers.
With various ships, islands, rafts, hidden treasures, etc the pirate theme simply seems to have more possibilities these days.
My hopes arent too high for the newest cowboy batch though.. it lacks structures. Might have been good if they skipped the fort for once in favor of something else. Like a ranch, or train.