A well-known class of constitutive relations for cohesionless granular materials are the double-sliding models. The double-sliding model proposed by Spencer (J. Mech. Phys. Solids, 12, 337, 1964) for incompressible materials as well as those of Spencer and Kingston (Rheol. Acta, 12, 194, 1974) and Mehrabadi and Cowin (J. Mech. Phys. Solids.26, 269, 1978) for compressible materials are generalized to describe the unsteady flow of cohesionless granular materials. These models are analysed and it is shown that they lead to equations that are not well-posed. Implications of this finding are discussed.