Hi Lewin_7965,
I understand Messages has not been functioning as expected.
Can I ask, are you using Messages in iCloud? If so, I would recommend going to: Settings > [Name] > iCloud, and disabling/re-enabling Messages in iCloud.
If the issue persists, delete the group message if possible, and compose a new messages to the group.
I would also recommend ensuring your software is up to date. You can learn how to do so here: Update your iPhone, iPad, or iPod touch, as well as restarting your device, which you can learn how to do here: Restart your iPhone.
Hope this helps!
Thank you for using Apple Support Communities.
Cheers!