Images not showing in Library

I’m new to Animate 2 - As per the video, after I selected the drawing to grab all the strokes, I dragged each new drawing (separate element) into the symbol library. The name shows in the library but the thumbnail is blank. When I drag one back to the timeline, nothing shows in the camera view or the drawing view. Why is this happening?

Right-click the template in the library and select the option “Generate Thumbnails”