How freeradius logs Accounting?

[Mark jensen]

I have configure free radius to authenticate users (using LDAP as back-end DB)

it worked very well but there is a logging probem:

the logged-in log:(radius.log)

Sat Aug 16 13:53:29 2014 : Auth: Login OK: [Mark/<via Auth-Type = EAP>] (from client private-network-1 port 0 via TLS tunnel)
Sat Aug 16 13:53:29 2014  : Auth: Login OK: [Mark/<via Auth-Type = EAP>] (from client private-network-1 port 5 cli F0-4D-A3-B1-34-16)

Accounting:
Sat Aug 16 14:02:56 2014
   NAS-IP-Address = 
   .
   .
   .
   .
   .
   .

why the start-time record is different from the Timestamp in radius.log, or in other words when accounting logs start?

another problem is when I logout as "Mark" and login as "Mike", it logs it successfully in radius.log:

Sat Aug 16 13:53:29 2014 : Auth: Login OK: [Mark/<via Auth-Type = 
EAP>] (from client private-network port 0 via TLS tunnel)
Sat 
Aug 16 13:53:29 2014  : Auth: Login OK: [Mark/<via Auth-Type = 
EAP>] (from client private-network port 5 cli F0-4D-A3-B1-34-16)
Sat Aug 16 14:06:26 2014 : Auth: Login OK: [Mike/<via Auth-Type = 
EAP>] (from client private-network port 0 via TLS tunnel)
Sat Aug 16 14:06:26 2014  : Auth: Login OK: [Mike/<via Auth-Type = 
EAP>] (from client private-network port 5 cli F0-4D-A3-B1-34-16)

but it didn't log any thing in raddact that according to Mike, even after a while?

what should I do?