Why aren t my text messages sending one my iPhone 5s?

I always text this person and my messages always go through ( bubbles are always blue) but suddenly when I send a message it say not delivered. Why? It only happens to that person, IIm able to text everyone else perfectly except this person

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  • Why is my iPhone5 sending some messages as iMessage and others as Text Messages? I understand that messages are sent as Text Messages when there's no signal or poor Wi-Fi connection. However, I'm successfully texting some people using iMessage but others (still iPhone users) as Text messages. The other person will send iMessages and then my responses will be sent as green Text Messages.
  • Why aren't my texts sending? For the last couple of days my iphone hasn't been sending all the texts. The sending bar will get almost to the end and it'll stop. What is wrong with it?
  • Why aren t my messages sending anymore? I have an iPhone 6 and have LTE and I ran out of data today and i have Wi-Fi at my house and all but when I left my house I tried texting my friends but iMessage won't send my messages and I tried sending regular text messages but those won't work either. I also didn't receive my messages till I got back home. Can someone help me with this problem please?
  • When I text other iPhones why is it sending as a text message? I have an iPhone 4 I was on AT&T and I just switched to straight talk. In my setting I have all the iMessage and everything turned on. Sometimes it will work whenever I text another iPhone but most of the time it won't. I have tried turning the phone off and back on. It's really irritating. If someone could help me that would be great.