Thumbnails in Lightroom Classic not displaying edits-history panel has edits-hitting most recent edit does display all edits- how can i get all the edits to be displayed automatically ?

If rebuilding previews does not work, then you may have to rebuild the entire cache by deleting it. Open your catalog folder and remove (or trash) ‘catalogname previews.lrdata’. Then start Lightroom. Initially you won’t see any previews at all, but then Lightroom starts rebuilding them so they’ll appear one by one. Lightroom will only automatically rebuild previews of photos that are visible in the grid. If you want it to rebuild all previews (for example overnight, so you don’t experience delays when going through the grid), then select all photos and use that build previews menu.

My guess — there might be a problem with the preview cache. You can try rebuilding it (probably best to try this first on one small folder to see if it works). Select the desired photos, then use the menu command

Library->Previews->Build Standard Sized Previews