ANSYS Meshing Problem

Hi I am currently using ANSYS 13 Workbench to do FEA on .igs files of leg bones (femur, tibia, and patella), I am new to ANSYS and I am having trouble meshing the bones. I am able to mesh the bones individually however when I import the whole leg assembly (assembled in proENGINEER) it will not mesh. Does anyone know how I can fix this?