Why doesn't chemotherapy always work to treat a patient's cancer?

There can be many reasons.
The big problem with cancer treatment is that the cancer cells are your cells. They are simply faulty; the normal limits that prevent them from misdividing are removed. Cancer cells are difficult for your body to deal with because they are difficult for the immune system to detect. These are your cells, which means they have your immune markers.
Chemotherapy drugs try to target cancer cells, but since cancer cells are your cells, anything that can kill cancer cells can also kill your normal, healthy cells.

The anticancer effects of fenbendazole are promoted by an increase in p53 expression. It is a protein that helps in regulating cell life cycles and preventing the formation of cancer cells in humans, which is why fenbendazole for humans is being used more and more.
Cancer cells divide and grow faster than healthy cells. This means that they can be easily starved out by changing their energy intake. Fenbendazole blocks the absorption of sugar and starves malignant cells. Thus, this dog dewormer blocks cancer in many ways.

What kind of medicine or supplements do you think are good for support in such cases? I would like to try something which I can get online from services like BidRx.com, for example. I would appreciate any input here, thanks a lot for your help and good luck!