If some people you are texting are not receiving your messages, this is probably because iMessage is turned ON. Indeed, if your group texts are sent with iMessage(in blue)
Then only iPhone users will receive it.
To make sure all your recipients receive your group texts, turn off iMessage (Go to Settings > Messages)
Before sending a new group text, you have to delete all previous messages you sent to the same group with iMessage ON. More information about this matter can be found on Apple website : Send a group message on your iPhone