Bruce thanks for you response, but the log size is fixed as 256 MB via domain policy for all servers, I checked it, it is not a problem.
After working hard, I found an interim solution, it was really annoying.
Our File Server is in "Server OU", and an object access policy was assigned for this OU; when I run the command gpupdate /force the command line says the policy applied successfully, however, in reality it didn't. I noticed this situation with running rsop.msc in file server.
To resolve the problem; I deleted File Server policy assigned to Server OU and then enabled "object access audit" in Default Domain Policy. This solved my problem, now I am looking for why the Server OU policy did not applied.
Regards,
Sahin Akkargan