fall flat
「江南スタイル」については当ブログでも言及しましたが，日本では海外のような盛り上がりはなく，セールスも芳しくはないようです。
http://d.hatena.ne.jp/A30/20121001/1349082826
http://d.hatena.ne.jp/A30/20121011/1349942407
fall flatは「（うつぶせ状態で）ばったり倒れる」で知られていますが，ここでは「まったく反応がない，少しもききめがない」という意味合いで使われています。こういうflatは強い否定のニュアンスをもたらします（cf. He gave me a flat refusal. adj.としての例ですが...）。
いずれにしても，時期が時期だけにタイミングがあわなかったことも一因でしょうね。（UG）
Japan's indifference to 'Gangnam Style' riles S Korea
While South Korean rapper Psy’s “Gangnam Style” has taken the world by storm, it has largely fallen flat in Japan—an anomaly viewed with grave suspicion by some in the singer’s home country.
Korean K-pop music is huge in Japan, but for one reason or another Japanese music fans have remained relatively immune to the seductive powers of Psy and his horse-riding dance.
In South Korea, there has been speculation that a bitter territorial dispute over contested islands may have caused Japanese fans to shun the song.
While the number has topped the UK charts and currently sits at number two on the U.S. Billboard ranking, it has only scraped into the top 30 of the Japanese iTunes chart.
To make matters worse, various Japanese music blogs have suggested that the video’s success on YouTube—530 million views and counting—was down to South Koreans using automated viewing programs known as “bots.”
Some even started playfully referring to the song as “F5 Style”—a reference to the keyboard key used to refresh the window of an Internet browser.
