If your cable and housing are not new, dsbrantjr's advice on replacing the cable is sound. If either are new, this might well just be some stretching of the new wire or compression of the new housing. (A lot less of that now with the new cables and housings, but it still happens.) Try turning the barrel adjust 1/8" of a full turn (to tighten the cable, in other words, the hard way). I like making barrel adjusts by easy to remember fractions of turns. Then I ride and see if I need more or less.

Ben