Not receiving all group texts

[AC Question]

I have a Galaxy S5 and I regularly have a group text thread going with some friends, who all have iPhones. I usually delete the thread after several months, but I'm not sure if my friends delete, or when. Very often, one person will send a text to the group and I don't receive it. Or, if I send a text to the group, one or more of them don't receive it. This happens too often to be a fluke. It hasn't happened with SMS that I'm aware of. Thanks for your help!