Hi piparoo, mof +
I did not study all posts. In post-2 sixpacmopar mentioned "clean the socket housing - where the nubs hold".
What I do: I take the bulb out. With a lead pencil "stump side / blunt-ended side" i push into the socket - I imitate "mounting a bulb". By this the pencil pushes a thin rod out (on the outside of the socket housing). With a brush of copper or of steel: I clean (brush) the thin rod and the "Mushroom-like end of the rod" - an a bit of the socket housing. Greetings Rolf