Why when I send a group text to more than 4 people do I get a "fail"?

[Android Central Question]

I just switched from and old LG3 to a samsung galaxy edge 7S. Group messaging worked fine on my old phone. I can't send a group text 2 or more to iPhone users and I can't respond either when any iPhone users are in the group. Went to my A T and T store, and they replaced my SIM card. This worked for 7 group texts, and then they started "Failing" again. Is this an iPhone Samsung incapability and if so, how can I rectify it? So frustrating!