I will reach out to the SME using their preferred contact method. If I get no response I will attempt to email the SME, Ccing their manager. If I get no response to the email, I will attempt to set up a meeting during a time that works for us both. If I am still unable to connect with a SME at that point, I will reach out to their manager. Perhaps they need the manager to give them capacity, or perhaps this is a trainable moment.