Disconnect the cable from the derailleur and operate the shifter. Does the cable move smoothly and easily when disconnected? If not, you may need to lubricate the shifter, replace the cable and/or housing. While the cable is still disconnected, use your hand to move the derailleur cage in and out. Does it move easily (taking spring tension into account) and smoothly? If not, the derailleur needs lubrication. Lubricate freely until the unit moves easily and smoothly. Reconnect the cable, check adjustment, and it should be ok.