I see what the problem is. You're using an AJAX update panel to update the hidden __PAGESTATEKEY input field - see attached screen shot. Test Studio load test cannot parse the contents of update panel responses in order to update the value of __PAGESTATEKEY for the next request. As a result all you will get is the original recorded value sent back to the web server.
You need to pull __PAGESTATEKEY out of the update panel and have it updated via full HTML page responses for it to be compatible with a Test Studio load test.