Why do about one in four text messages I send to my niece disappear?

It only happens when I send text messages to her, no one else. She has reported it to Cricket which we both have and they say they are investigating it; the investigation has gone on for 4 months she tells me. She says these things take time. So far, they have no explanation.

What could be the problem? It has made it very hard to communicate by text which, for me at least, has been the easiest way due to my bad hearing. I just got hearing aids but am still not used to them and generally I prefer text messages anyway. But losing every 4th or 5th message is frustrating and has led to a few misunderstandings in our attempts at communication.