The potential of cancer stem cells to function as tumor initiating cells has been debated. The heterogeneity within a population of CSCs might be adding to the complexities; some drive tumor formation while others metastasize. However, all these mechanisms are interlinked and CSCs are cells with various capacities; self-renewal, tumor initiation, differentiation, quiescence, chemoresistance and metastasis. So, it is very likely that the breast cancer stem cells you are working with, are capable of forming tumors in nude mice. The load of viable stem cells injected for tumor induction might be a key factor as far as the technical aspect is concerned.