Group iMessage Chat changing to SMS and won’t revert to iMessage

I had a family group chat that contains seven people including myself, all with iPhones ranging from 13PM to 16, all with the latest iOS and a few with iOS26, the original group chat was active for two plus years.

Within the last three to four weeks, the group chat changed randomly one day to SMS, no one had switched iMessage or iCloud off or changed networks etc it was just random and it never went back to iMessage.

When messaging the members individually all of them had iMessage chats but the group did not.

We created a new group that same day and it has been fine, until this morning, when the group has changed again to SMS, only this time overnight and hasn’t changed back to iMessage. I have looked again to check and all contacts have iMessage enabled and I can iMessage all of them individually just not on the group again.

I’m not sure what is going on or what to do about it so any help is appreciated, I don’t want to keep creating group chats.