If adjusting the outer limit screw doesn't help (and if the derailleur hanger doesn't appear to be bent inward), replace the skipping cog. And from now on, don't use the smallest cog except with the large chainring. Riding around using the smallest cog with the small or middle chainring wears the cog out fast. For the equivalent gear ratio, use the big chainring with one of the middle cogs on the cassette.