What are the causes of failure to induce Leukemia in recipient mice?

A biotech company discovers a new drug that affects the stability and activity of BCR-ABL. You want to test its effect on Chronic myeloid leukemia cells. You got interesting preliminary results when you treated CML cell lines  in  vitro  and you want to move one step further to test its effect in vivo on the CML mouse model. You read the article entitled "Establishment of a murine model for Therapy-treated chronic myelogenous leukemia using the tyrosine kinase inhibitor STI571" and you want to reproduce it in your lab. You did exactly  the same steps that were described in the materials and methods to generate this mouse model. However,  the mice did not develop leukemia and did not die. In your opinion and knowing the different steps needed, what are the causes of failure to induce Leukemia in recipient mice? Discuss the optimizations experiments that you propose to do to solve this problem?