I switched from ATT to Sprint last year. I never had any issue with ATT for sending or receiving group messages. But in Sprint I do have an issue with sending messages. I don't have any issue in receiving.
I have iPhone SE. Again no issue with receiving messages. When I send message to multiple people (group) sometime it does send immediately but some does not send message and says "not delivered". Do you know why am I facing this issue? Any setting which I'm missing? Response appreciated.