I am guessing that the shifter is fine and may have been set up improperly.
Cycle the shifter untill the max amount of cable is extended (hit the side tab 3 X)
Then install the cable again with the DR on the small chainring (should already be there)
Make sure your high limit screw is not blocking the travel of the DR.
Try again.
If problem persists you have a bad shifter.
Enjoy