I felt so stupid when I found out that "Setting" was the reason for not generating the log file.
1. Of course, Settings - Document Properties - General --> "Generate Logfile" should be checked.
2. [I don't quite understand why, but this was the reason for not generating the log file]
In Settings - Document Properties - Security --> "Admin Override Security" should be checked.
Normally we uncheck this for others not to configure the QVW file.
I might have missed this part.
Anyway, please share your issue or other solution for this kind of problem.