Just a quickie. I've saved two materials as assets. One, Speckled Ceramic, generated itself a thumbnail. The other, Smooth Ceramic, didn't. Would you point me in the right direction. Thanks.
Just have to wait. Depending on your machine specs and whatever else you are doing in blender it can take awhile. Blender sets it to a low priority. So even on high-end machines it can take as long as 3 minutes. Also it helps to set your GPU up in preferences->system. You can sometimes speed it up by selecting the asset in the asset browser and on the n-panel click the recreate thumbnail icon. Sorry I don't remember what it looks like off the top of my head. If you go to the preview panel if the material tab in the properties editor and wait until that generates before marking a material as an asset it takes less time to generate the thumbnail.
