Unfortunately that's the nature of feline leukemia. We had a couple feral friends who were the sweetest cats. They both were fiv positive. They were normally fine but would go through bouts of sickness getting weaker all the time until they finally succumbed to the insidious disease. My prayers go out to your little friends. They don't deserve to go out that way.
Also, the disease is extremely contagious. Do not mix them with non fiv cats as the disease is spread through bodily fluids. And be sure to wash your hands after handling them and before handling your non fiv cats.