Loosen each dull string, one at a time, until you can pull it up from the fretboard about an inch to an inch and a half. Let it slap hard back down on to the fretboard, repeat this moving your lifting position up and down the fretboard about 20 times or more. Then retune that string. It will then be brighter than before. Compare to the brightness of the other strings before moving to the next string.
I think it has nothing to do with removing dust/grime, it's more to do with relaxing the outer roundwound wire from the central core.
Then repeat for the other strings.