Hi Robert,
It sounds like the private key was not included in the export of the certificate. Here is the official procedure:
On the system that has the ISV certificate that was published to SCCM:
1. Export the eXc Software certificate as a .pfx file – include the private key in the export
On the system that will use the common ISV certificate
2. Delete the existing certificate from the local key store
3. Copy the above exported file to the system
4. Import the above file into the local key store
John