1. This can happen when the certificate is installed without having logged in as an administrator. You should always log in as an administrator and not as a user without permission.
2. Another possible cause is that the correct file has not been installed. The certificate file must be the one downloaded from the Camerfirma website and has an extension p12. Only that file is valid. If you do not see the file extension, you can see it in the file properties - place the mouse pointer over the file and click on the right button. If you try to install the certificate with a .cer file extension, the installation will not work and the certificate will not be installed in the Personal file.