Recently we found that if you are using the updated ULS Viewer (16.0.3129.1000) from Microsoft (https://www.microsoft.com/en-au/download/details.aspx?id=44020), and you have changed the default logging path in Central Admin, then you may find that the real-time monitoring in ULS Viewer does not show any log entries. Opening a specific trace file however works fine.

Changing the logging path in Central Admin back to the default value and real-time monitoring in ULS Viewer starts to work again. This appears to be a bug in this version of the ULS Viewer.