if the problem is the "pawls" in the shifter ratchet not engaging (characterized by no resistance/noise/tactile sensation when the lever runs through its range of motion) then a simple solution may be to spray some form of degreaser (tri-flow also works very well) into the shifter. repeatedly work the lever putting a slight force in an orthogonal (perpendicular to the plane that the swept motion of the lever is in) direction, this may result in the eventual breakdown of the coagulated grease that is hindering the free-ratcheting motion of the pawls. this is the ONLY "service technique" that Shimano ever discusses for any of their shifters. if this does not work step two is to replace the defective unit.
dura-ace has a three year warranty from date of purchase.