What are the chances that a sent text doesn't actually get sent to the person at all?

Most of the time when I text this person. I never get a text back and he says that he never got it and doesn't know what I'm taking about. then there are times when I will text him and he will text me back right away then I will text him back again and I will never get a text back. I mean how are you suppose to progress things with someone when that happens. I'm not sure if I believe him because he's the only person that does that to me or that happens with. What do y'all think?
What are the chances that a sent text doesn't actually get sent to the person at all?
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