Discussion items in a group are no longer showing

I have a discussion going in a group. A couple of people replied yesterday and I also added a comment. This is all shown on the main stream (see disc01.jpg). Today another person commented but when I click to go to the discussion, the only comment I now see is the one posted today (see disc02.jpg). I tried clicking on the links in the stream for yesterdays posts but that does the same, takes me to the discussion but only the one item and one participant (other than me.)

I'm not sure what's going on here and why the previous people's items no longer display in the discussion thread and are not shown as part of the discussion?

(On the back end I see their posts listed. )