Why does my mother in law never want me to have a break?

Why everytime I take a day off to get things done or to do something I want to do my mother in law will refuse to watch my kids but if my husband takes a day off for the same reason she will bend over backwards to make sure he is able to be alone? What am I doing wrong? I took off today to go help my sister and my mil said when she got up she would call me to bringthem to her and its almost five at night still no call.