I am modeling a simple suspension system. The two bodies are modeled with discrete rigid shell elements. A point mass is added to the reference point of the upper rigid. The lower cube is constraint in all 6 DOFs at the bottom. 12 axial connectors are defined between the two bodies to model some springs. 
However, when I tried to conduct a modal analysis, I got the following error:

Three factorizations in a row failed. Check the model. It is possible that the model contains the kinematic coupling definition set up in a way that a degree of freedom has neither mass nor stiffness.

It seems that my mass matrix or the stiffness matrix is wrong. Would someone kindly point out where I made a mistake or missed a step? Or is there a tutorial with connectors between rigidbodies? Many thanks.