Correlation Rules - Not Applied to web_add_header function

In my application, there is a header "key" which is sent for every web_url request. For example,

web_add_header("key","quwjkajhsiduyqiwhdkjhsduq")

web_url(.....

I created a correlation rule for capturing value of "key" but when i record/regenerate script, it does not replace value of key with correlation parameter. I am very sure that my rule is correct as i tested the rule. Please help on this.