I'm having a similar issue, and one that I see described in a number of forum threads. As described by the OP, a working SBSAR file is loaded by the Maya LT 2018 Substance Plugin inside Hypershade (plugin version 1.2.4.0-bb5eb94 in my case), and nothing updates or responds to clicks. Sometimes the "Reload" button will add the absolute path to the substance name.
No inputs are shown in the window area below the "Reload" button. Pressing "Create shader network" creates an error popup that says "No Substance Output Node could not be automagically connected to the shader node. Pleasemanually [sic] connect the output to the appropriate shaderinput or exit custom workflow". Side note: the popup mentions "custom workflow" even while in "StingrayPBS" workflow, and actually the "Create shader network" button is not available in the "Custom" workflow.
A handful of downloaded files from Substance Source work, but not all, and none of my custom Designer files do (they all use the standard metallic PBR outputs from the included template, some with AO).
Any advice on what the specific requirements are for the plugin would be appreciated. Also, since I was only trying this because I thought it might be easier than creating new basic materials and loading the basecolor/diffuse maps to do some model tweaks (e.g. no baking or rendering or anything that actually needs the full PBR material), would it be better to skip the plugin entirely?
Thanks