Emails in Conversation

Dear support,

I have an issue recently after install version 7.2.40748.0.
For mail sent by me and replied by others, even I selected the conversation option, occasionally some of the replied appear as separate item in the list view, so I lost track of the whole email series on one topic.By right it should go into the original line in the list view and only increase the number of emails counts.
Any idea what happened and how it can be fixed?