My low wall had the same problem.  After reading these posts I decided to pull the forearm and check it out.  Without the forearm on it functioned as it should, good and tight.  After inspecting the forearm I saw that it was making contact with the mainspring screw. and had worn a shiny spot.  I removed a small portion of the wood with a drill a little larger than the screw diameter.  I reassembled it and it is good as new.  I might have gone to a gunsmith to correct the problem but you good people saved me some bucks.