First, adjust the rear der cable tension. 99% of shifting problems are just cable tension adjustments.

If the cable tension is good, shift down to your smallest cog in the back. Take out the rear wheel and pull the shift cable by hand. The der should move smoothly and easily back and forth as you pull on the cable. If there is any binding, release the cable and push on the der by hand to make sure the der itself is not binding. If the der is ok, replace the cable and all the cable housing.

If after replacing the cable and housing, there is still a shift problem, then douse the shifter with WD40.