I don’t get it?

So my bf is in basic training. I’ve been sending him letters and he is telling me he hasn’t gotten any. I put the correct address and everything and nothing. And they haven’t come back to me so I know it’s right. I feel bad because I don’t want him to think I went ghost on him all this time I just dont know why he’s not getting them 😞. And I even tried Sandboxx and they say they are delivered. He doesn’t have that long yet but I don’t want to just MIA like that..