EXM dispatch Server | Dispatch server is not working after second thread of emails


I am using sitecore8.2 update2. I have created dispatch server. Sitecore sends email by dividing in multiple threads. Dispatch server is running fine for 1st thread and sending multiple emails. but when 2nd thread starts is started throwing a error in log file "Jobname Sending message (D0983D6C33974894A99DD9430272C7F2) not found". the given id is campaign id. which sitecore founds for first thread but not finding after 2nd thread. what could be the issue.