iMessages sending as text messages for only one contact

I had this issue too. This is what worked for me. Instead of messaging in the existing conversation, touch the icon to type a new message. Start to type the person's name in. In the list of suggested contacts, on the right side, there's a little arrow. Touch that arrow to expand all the contact methods for that contact. Notice some are green and some are blue. Choose a blue one that's not the same number you've been sending to. (For instant, my contact had an email I could choose which was blue.) Send a message. Then start over, do the same thing again in another new message, and this time choose the original number you were texting to before. Hopefully it's blue now. Send a message to make sure.

This worked for me but have no clue why it just started doing this and I had to change it when it always worked for me prior but thank you