How to detect http response body

[Maxim <hittlle () 163 com>]

Hi all,
We came across a weird scenario. We used the following rule to detect http response body
  alert tcp any any -> $HOME_NET any ( sid: 10000003; file_data; content:"ASED"; nocase; rev:1; classtype: 
misc-activity;msg:"cve-test";)
And we do see such a string in the response body, but snort did not trigger any alerts. Are we missing anything? Many 
thanks

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