iMessage not working for one contact
I have an iphone 6s. Suddenly I am unable to iMessage my daughter who has an iphone 6. I can only text message her and then I am not able to see her reply on my phone. I am able to see her reply—an iMessage on my iPad. I checked to see if I blocked her by mistake. I turned iMessage off and then on again, I did a hard re-start of my phone and none of this worked. I also tried going to send/receive in settings, messages and using my email, but it still stayed green, a text message. None of these worked. Any suggestions?
iPhone 6s, iOS 11.3.1