I totally understand the lack of response being frustrating, but like others said, it could be her upbringing. My family ALWAYS responds with something. I'd probably call her out on it. If its that frustrating, it'd drive me nuts till I found out the reasoning behind it.

I would never call my MIL 'mom' either. I won't even call my dad's 2nd wife my 'stepmom'. Since they divorced when I was 16 and he didnt remarry until a couple years later, she's never been a mother figure to me. Maybe if my mom wasn't such a big part of my life, and my MIL was a mother figure to me, then I'd consider it.