Women’s button down pajama top

I got myself my first set of the women’s short sleeve and shorts, button down pajama set. I’m wondering if it’s a sizing issue or just happens to be a bad set I have, but my buttons come undone ALL the time. If I’m awake moving around, they pop open, if I’m sleeping I wake up to only the very bottom one still closed, the rest are popped open. Does anyone else have this issue? Is it just this pair? Are they too big?