What are reasons that a 19 yr old all leukemia patient who achieved remission through chemo be unable to have it again when he relapses a year later?

Toxic or no effect: It is possible that someone can have significant side effects for instance damage to the heart that could decrease the ability to receive the same type of chemotherapy. In addition it is possible that cancer cells may be resistant to the same type of chemotherapy. Individual cases would have to be discussed with the physician.

Chemo should work.: Certainly this is a bad situation, and I am sorry for you. But this is fairly common. There are well established protocols for the treatment of early relapses of all. I would get several opinions, if you have been told that treatment options are limited.