For me and my son it was his app. I don't recall what app he liked at the time but he was not receiving my MMS messages to him. I believe he could send MMS to me, I don't remember that being a problem, but he was not receiving my messages to him. After some texting it turns out it was the app he was using. You can check this, because I'm pretty sure we found the messages in the stock app. So open the stock app, it of course will ask you if you want that to be the default, answer however you like, and check if the message is there and I believe you will find them.
I don't know why the app he picked didn't work, and maybe if she really loves it and can't live without it you can update your question to how to fix (blank) SMS and someone will have the answer. As for our issue I just had him change apps.