Issue with replies opening a new ticket

Issue with replies opening a new ticket

we are having an issue since upgrading to version 8.  We have all the latest service packs now loaded.  Whenever we reply to a request for more information the user replies to the email they get, instead of it going into the conversation of the already open ticket, it creates a new ticket with their reply.  This was working completely fine before the upgrade
 
any ideas?
 
 PLease help
 
Version 8.0.0 , Sp 8003

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