Sorry if this has already been addressed but I've been through the ringer with this problem. Went from iPhone to Galaxy 5 in November. Mostly no issues except for my messages. Came to find out that I'm not receiving some people's texts, mostly group texts from what I can tell. For example, I sent a message to 4 people, all iPhone users. I can only see responses from 2 of them even though all 4 are responding.
I've confirmed that my iMessages was deregistered, had them delete previous conversations, and deleted my contact from their phones as recommended. I have also tried several types of text apps including messages and messages+. Still the same results. Spent countless hours on the phone with Apple and Verizon but have gotten nowhere. Anyone have any ideas (other than giving up and going back to Apple)?