I've been having this same issue on outbound text (only) with my i6s to a group and it probably happened on the latest iOS upgrade as best I recall it becoming troublesome(?). I have a service ticket in and have been in touch with Verizon several times, even turning the SMS on which I don't like to have enabled - I prefer keeping it iMessage to my apple family/friends, but that didn't help either...Enable LTE was set to 'data only' and changed to 'data and voice', reset network, restored i6s to factory and restored to iTunes backup and it still fails with group texting...mostly AND also, on my first bubble text out to the group seems to be the highest failure rate. it does fail later in same group and it's even failed on an individual text. I'm at the point of getting Verizon to send me a replacement i6s now! if fails on small groups too with mixture of apple and droid cells.