auth_log problem

Hi I’m using FreeRadius 1.2.12 for about 50.000 concurrent users coming from about 50 NAS. I found that More than 40% of radius packets (1812/1813) was seen by tcpdump but not seen in auth_log or acct detail log files. What’s wrong? Does freeradius have limitation for such case? radiusd.conf is as follows: start_servers = 5 max_servers = 128 min_spare_servers = 3 max_spare_servers = 128 max_requests_per_server = 0 max_request_time = 60 cleanup_delay = 5 max_requests = 256000