Ah; I can now get a better view of the problem.

The ability of members to edit posts that have been "published" on a group's web UI and by email to the group's members can be disabled by an owner (or moderator) as one of a group's settings, and I can only conclude that that is what has happened in the group where this problem has arisen. When disabled, the Edit Message tab In the "More" hamburger is not available.

The facility of editing messages once they have been posted has its uses, but it must be used with considerable care because if a message is changed after others have responded to it, it may well be that those responses are to something that is changed in - perhaps even removed from - the original message. The result can then be that the thread of the messages ceases to make any sense at all.

Perhaps the owner / mloderators of a group where you can no longer edit messages once posted has decided that such editing is corrupting the "flow" of whatever the topic is and has thus decided to stop members being able to edit messages that have been sent.

It is not a fault of any sort, simply the change of a setting that has disabled members' ability to edit messages.

Chris