Sudden inability to reply to group chat on iPhone only

We Have a family group chat that has been in place for well over a year. Yesterday, I found I could not respond in the chat - the ‘+’ sign at the bottom wasn’t there. I can see all incoming messages, and I can reply on my laptop. Nothing changed that I’m aware of.

Very odd - and all in the group are on iPhones and they can all work the group fine. I have rebooted and tried quitting iMessage, etc. No joy!

Any ideas?