Asterisk Does not clear Channel after a BYE

I’m new to asterisk.

I’m using asterisk 1.8.1.1,

I have a simple dial plan to connect two peers. where the peers are two different voip applications.

when the call comes in from one application, Asterisk connects to the other peer. when one of them hangsup, asterisk should hangup the channel.

the scenario works fine for few calls. But when i ran a bulk call with SIPp. the following happens:

peer1 --Invite- > asterisk ----INVITE–>peer 2

RTP

peer1–BYE–> asterisk — BYE----> peer2
asterisk <—200OK–peer2
asterisk -------BYE —> peer2
asterisk <------200OK–peer2
-
-
peer1<—BYE----asterisk
peer1-----200OK-> asterisk
peer1<----BYE------asterisk
peer1-----200OK—>asterisk

this is making long silence on the channel. I have no idea why asterisk is not hanging up the call.

Regards
Gnaneshwar