Get a look at the switch contacts with the field as flat as possible, or with the spinner held perpendicular to the playfield. Now rotate the spinner and make sure the moving contact touches and slightly pushes the stationary contact. Adjust it so it has the same throw as the other spinner.
Inspect the contact points for damage. Over the years the gold flashing can get "smudged" so that the contact points are irregular. At that point the contacts are at end-of-life and should be replaced. You can try to smooth them out with 1500-2000 grit sandpaper. Note that you should never file or sand gold contacts unless they have end-of-life surface damage.