Just assuming and don't know anything for sure. It may be possible that you can't resolve this issue on your phone alone. You may have to make sure that everyone in the group has their phone turned on for group messaging. Friend 1, 2, and 3 might have group messagin turned on but friend 4 does not. So when Friend 1 sends a group text to 2, 3, and 4 they will receive it and reply. The replies would come back with 2 and 3 included in the group becaus they have group messaging turned on, but when friend 4 replies, it goes to an individual text because friend 4 does not have group messaging turned on. Just a guess, try getting together and setting all the phones to allow group messagin and see if that works, but what it boils down to is you can't control what others phones are set to and how they respond to your group message.
Personally, group messaging annoys the crap out of me on my phone so I will start a whole new message and just send to the one person I'm talking to and stay out of the group (keeps me out of trouble too).
Hope this helps you guys to figure out a solution. I'm having the opposite pain of tryin to make sure group messaging is completely turned off so I don't have to start a new message every time I get a group message.