All of a sudden, one person I text who is also an iPhone/iMessage user, will only send as a text message. I can text any other iPhone contacts and it will send under iMessage. This persons texts that he sends to me pop up on my phone as iMessage so I know it’s nothing he has done.
iPhone 5c, iOS 10
If you go to Settings>Messages, do you see any errors to your iMessage activation? I would still tend to believe it is a problem on the receiver's end. Ask if they are connected to a stable internet connection? If not, then it can affect iMessage. Both the sender (you) and the receiver (them) have to be connected to a stable internet connection for iMessage to work properly. Also check you Send & Receive setting.
Imessage
