Issue with Apple Mail Conversation

When I reply to an email within Apple mail, while using the same server (ie: Google, or iPage, or iCloud, etc.) the reply will not show in the conversation of the original email as a thread. The most perplexing part of this issue is that it is sporadic. I just sent an email and it did appear above the original email as a reply within the same email as a conversation/thread. Other times I will send an email and it will go out (it is not in the outbox, rather it is in the sent section) but it will not report within the email as a thread.

I have marked "Organize by content" under the View drop-down of Mail ap.

It is quite frustrating that it happens occasionally. It would appear that I did not reply to an email when it is not showing as an email thread so, I often find myself writing the person over and over again. Going and checking the Sent section of Apple Mail is tedious and a waste of time when trying to determine if I sent an email reply or not.

Please help, it is quite frustrating.

* Running High Siera 10.13.3

Thank you,