How do I switch from text to iMessage For one person

Hello! They definitely have iMessages enabled because we’ve always been texting through iMessages. I only started seeing the green bubbles after attempting and failing to “send as text message” once by accident in a family group chat of three people. Since that moment, I always see the green text bubble when texting each of these two people or in our group chat of three. I still text others in the blue iMessages with no problems.

The option showed up when I couldn’t send 20 photos through iMessage and the app asked me to resend again and again with alternative options of sending as text or with Personal. While I was trying to resend this, I could perfectly still send them other messages and other photos and receive their replies. And at one point the resend request becomes too annoying so I decided to send as text. Since then the problem of green bubble started. So I think the problem is caused by me switching to text messages because before that I could perfectly text them through iMessages.