Usually this issue is due to the fact that VS is not able to extract the proper dynamic parameter.
At record time, the value of each form post and query string parameter is recorded, and an appropriate extraction rule is computed. Immediately after recording the web test is played back by the correlation tool. During playback, the tool applies the extraction
rule and compares the extracted value with the recorded value. If they differ, it flags the parameter as a dynamic parameter. However, there is one situation that it will work well, for example, the session id, usually server will provide one session id for
a few minutes, if you run within this period, the web test should be fine, but when it expires, the session id recorded will not work any longer, you should extract the session id field yourself manually and bind it to the requests.
Here are some more articles about the debug technique you can refer to: (including VS2008 and VS2010, which are all helpful.)
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