I haven’t spent a lot of time diagnosing it but it seems to me that you can’t use the @ with OP’s ID until they have also posted a response. The other responders are available in the list. Like I said I haven’t spent a lot of thought on it but, right now, all of the responders in this thread are listed. If true, then maybe the programmers just load the responders’ IDs to the list.
Wow, I tried to add this as an additional post right after this one but it wouldn't let me post it! Here's what that additional post would have said. :)
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And now that I've posted, mine is available as well. I'd bet there's an
upper limit to how many are shown though. Can you imagine the list of
IDs for some of the longer-running threads here? Maybe they limit the
list to responders in the last xx hours.