Outlook 2007 Conversations split

We use Outlook 2007 and Microsoft Exchange 2007 and have users all over the world.  The issue we need to resolve in some way, shape or form is that, from time to time, someone starts a "conversation" and replies show up as a different conversation.    Said another way, I send an email to many people in the company entitled "business forecast" - and in the conversation view I have 2 conversations by that name - my original, and their responses.  The "related messages" function fails to link them, even though every response has the original in it.  The does not happen all the time - it is the exception, not the rule.   Where should we look to trouble shoot this?  
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Check that Windows Search 4.9 is installed:

Should be

Check that Windows Search 4.0 is installed:
Related Messages is associated to Windows Indexing functionality within Outlook
Check that Windows Search 4.9 is installed:

Then try re-indexing via
Start>Settings>Control Panel>Indexing Options>Advanced>Rebuild

When re-indexing, make sure Outlook is open and all subfolder as well as any personal folders or archive psts.

This should sort it for you.

Also, would check you have the latst SP installed:
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