I think I have set the settings for messages (including MMS) correctly on my devices but something new has recently popped up. If I send a group message with a mixture of iPhone and droid users, it comes through as a single message to the user and the reply back to me. I’m not sure that makes sense, but maybe someone can help.
Why must people insist on embracing the dark side and not using an iPhone 
Group MMS is not very clear, but I think group MMS is partly controlled by your carrier and partly the recipient’s carrier. Group messages default to SMS (which is single sender to single recipient) if you can’t use Apple or MMS. I think your phone basically triages and goes Apple? Nope, MMS? Nope, SMS it is. I don’t know how you troubleshoot if an MMS problem is with your carrier or your recipient’s. For your own carrier, you can google your plan to find out if it includes group MMS, but you can’t do that for all your friends’ plans. This is what WhatsApp was invented for 
Out of interest I’ve just googled Android settings for this and there is an MMS/SMS group text setting that can be set at a device level regardless of carrier. It might therefore be that a recipient you are texting has MMS turned off by default and all group messages are being converted to SMS.
What settings do you have enabled under settings > messages? There’s a checkbox for mms
OP says they’ve done their own device MMS settings. But you can’t control the settings of recipient devices and they might have MMS turned off.
Big fan of What’s App. . Not sure I can get the others to embrace it.