Not receiving some texts on my galaxy s6 on AT&T?

[AC Question]

I'm not really sure who's texts I'm not receiving besides my sister's, and it is kind of hard to test it. But for example today, we were having a conversation and i randomly did not receive two of her texts, which i only knew since I asked her to ask again if I don't reply. We are both on AT&T and she has iPhone and I have a galaxy s6. Again, I'm not sure if it is just her or if is all my contacts.