Why do I get dropped from group text with iPhone users?

[AC Question]

why do I get dropped from group texting with iPhone users?

I recently started getting dropped from my group texting thread with iPhone users. It's with my Samsung Galaxy S4 and the 3 others have iPhone
s. Only one of the 3 got a new phone. When we restart the thread we are all on. Then I get dropped. They all can see me but I cannot see them. Can someone help? Thanks!