Patio houses, nuevas obras

Has anyone got experience of trying to get their property inscribed in their names in the Registro de Propiedad. In our case a name was recorded 70 years ago, this was the last time a name was registered. Since then the property has changed hands many times by private contract. We are now unable to register the property in our name until we get a notarised signature from the descendants of the former registered owner. This is proving impossible! Any advice would be really appreciated.