When you reply to someone, use the reply buttonXbox is open
OkWhen you reply to someone, use the reply button
But I don't remember exactly what you need to do, but you'll find the answer from here.
I remember seeing threads before where people ask this, and they have gotten an answer what to do.
So maybe try the search function?
And a tip, the threads are most likely in the Xbox support archive so search from there.