Visual Studio Web Performance and Load Test: Cannot save data to database during load test

I created a Web Performance and Load Test Project in MS Visual Studio Ultimate 2012. I added a Web Performance Test which recorded steps to register new application in my website. After completion, I checked that all data were stored in database (SQL Server 2012) successfully. Then I try to test it again using Run Test. The result is passed, but I found that some data cannot be stored in database.
The Application data is divided into 2 parts (basic info and subjects) and stored at 2 different tables. The missing part is subject table
It will pop up a new windows when user click "add subjects" button in the application, then user select subjects and click select button in the pop up windows. The windows close and all subject data will be saved in session state. After, the parent windows refresh and load subjects. User confirm it and click the save button finally (The subject data will be stored at DB from Session state).
1) I think th ...