Unable to send group text to specific contacts on iPhone

I am having a problem sending a group text to 2 of my contacts who I have sent to before. I can reply to and send to other groups already set up that includes these 2 contacts. I can send to these 2 seperately. I can create new groups with other contacts. What could possibly the problem with sending to these 2 contacts in a group text?

[Re-Titled by Moderator]

Original Title: Group chat