iPhone 6s not recieving certain text messages?

So my boyfriend is the only person I text who doesn't have an iPhone. He has a Samsung galaxy.. (I'm unsure of which one) and when we're texting sometimes, we discovered that he'll send a text and I won't get it, so thinking he didn't answer I'll just leave it and it's extremely annoying. We figure out what happened and keep texting and about a day afterwards the text goes through, as if he just sent it. I'm not sure if it's my iPhone or his Samsung that's the problem but how can it be fixed?