The advent of the Web of Things as an application layer for the Internet of Things has led to widespread of Web services exposing data and functionalities of the networked objects. Since resource-oriented architectures align well with the Web of Things archi-tectures, RESTful services have been the goto interface to expose the connected devices on the Web. However, the heterogeneity of descriptions of devices and services as well as underlying IoT-level protocols has lead to a number of interoperability issues. Recently, the growing popularity of semantically-enabled Web services has led to the emergence of services described with and exchanging RDF. In this position paper, we attempt to frame the challenges encountered in enabling semantic interoperability of heterogeneous WoT services, with the help of a production line scenario. We also propose preliminary solutions to the main issues hampering the establishment of true semantic interoperability on the WoT.