Alzheimers is never the same day to day or hour to hour. Just roll with it. If your mom is lucid, stay and talk with her. Let her thoughts drive what you talk about. Tell her you love her. If she is not having a good day, do your best to provide comfort and understanding.
My son said it best. Grandpa (my father) may not know who we are or know why we are there to visit. But if we leave him feeling better than we arrived, his brain will release "feel good" chemicals and so it is never a wasted visit. My father passed a year and a half ago and we treasured our last few years with him.