What are the reasons of error "Cannot read outputs from Prime worksheet" while doing Creo-Mathcad integration ?

Hi..

I have created one design in mathcad which having some inputs from creo and one output from mathcad.

So when i am going to do integration some times it gives me the error "Cannot read outputs from Prime worksheet" which i am not able to get the exact reason.

Please can some one help me to understand, so i can take precaution while doing integration.

Thanks,

Prashant Pandarkar