The Tzaddik said that my mother’s disease will disappear, that she will be healthy. She is worse. What happened?

First I want to wish your Mother a Refuah Shleima - may Hashem bless her with good health. I don’t know the Tzaddik you went to see and you should really ask him for a better answer. The way I understand it is that the Tzaddik prays for your mother's health but no one can guaranty us health. Not even a great Tzaddik. We can ask and beg Hashem, we can do good deeds and give Tzedakah, we can repent and say Tehilim – all that is good and helpful, but Hashem is to do what is good, though sometimes we can not see that good, we believe and know – Tov Hashem Lakol – Hashem is good. Kol tuv