request timeout

the Lifetime max session is 770 in my 4500 whose licensed capacity is 2000 .

however , one of 17 session agents whose address is 10.90.166.227 can't  make inbound call .

I got the siplog on the session agent found that the 4500 doesn't respond to INVITE request , after 10th retry (request timeout ) the call was cancelled automatically .(sometimes there got a 200 OK response from 4500 after the while Cancel has been sent to 4500)

As there are 109 active sessions on the 4500 so I got the log.sipd  and there display that

"

[MINOR] SigAddr [SouthBase-ProvinceBranch:10.90.166.227:0=low:NONE] ttl=86400 guard=59 exceeded message threshold of 4 " .

 

what's the reason that  all INVITEs from the agent 10.90.160.227 are  ignored  by the 4500 ?