I can see only 2 cases when that can happen:
1. When the mail clients of your correspondents do not use standard headers "References" and "In-Reply-To" (as discussed in this thread) or
2. When they modify the subject in the way that is not known by Aquamail [Aquamail knows and disregards common prefixes and suffixes in different languages, such as Re:, Fwd:, (fwd), but that list might be missing something, especially if it is not in English). -- This would happen only if you have the option "Break a conversation on Subject change" enabled.
The subject is not modified in any manner with suffixes by their clients, this I just confirmed by looking at the emails again. In fact in Gmail app it has never had any problems with conversation view with these same group of people I have corresponded with for over 10 years. I confirmed by looking in my Trash and they are all grouped properly as one would expect. I also just checked Aqua mail and looked in the Trash folder and the subject is the same for all the responses to that email thread. So, I suspect, Paris Geek asked you to post the screenshot to see if there is any subject modification. I actually thought of asking you if you have "Break a conversation on Subject change" enabled or disabled.
The options I have set in Aqua is "Combine on sender + subject" although I tried all the other options it made no difference.Personally, I'd suggest you looking at the headers of the messages in question, and checking if they have those two headers. (You can do this from the message view -> 3-dot menu -> View -> Show headers, or by check-marking an individual message in the message-list-view and 3-dot menu in the floating bar [if your configuration/version has that] ->"Show headers".)
Typically, those headers would be toward the bottom of the headers.
If those headers are absent, then the culprit is clear, as described previously here. (And then you can complain to your friends for having a poor taste in e-mail clients.)
If they are present, - we can try to help you figuring out why the messages are not linked into a single conversation in Aquamail.
I checked the headers for individual emails in question and I have attached them here, I edited the emails and names but nothing else has been modified. They all have a "In-reply-to" header and reference although the header sizes are different.
I'm hoping this helps solve the mystery. Two of the 3 have this in the header, but again the same emails are grouped properly in Gmail.
"In-Reply-To: <CAM4Sxfj+NZjkO9sB=2a6dZN4Kgs3FF9-Q+7f+XyMYGkw3N9kXg@mail.gmail.com>"
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Subject: Re: RMZ
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From: J H <j@hotmail.com>
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Subject: Re: RMZ
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<op.yu61x5jinwub7b@john-pc>
From: A Y <h@gmail.com>
Date: Mon, 6 Feb 2017 00:15:46 +0800
Message-ID: <CAM4Sxfj+NZjkO9sB=2a6dZN4Kgs3FF9-Q+7f+XyMYGkw3N9kXg@mail.gmail.com>
Subject: Re: RMZ
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Cc: D L <rr@aol.com>, A Jet <a@gmail.com>, JIM
HOULIHAN <j@hotmail.com>
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