Make sure the shifter is not rusted or stuck inside. Spray some gobs of WD40 in there, let it soak, and click the shifter a bunch, upwards and downwards, wiping away excess gunk that comes out. I had shifting problems on an SA AW recently, and the shifter was getting stuck between gear 2 and 3, even though the cable, etc. were fine. Also might consider swapping in a replacement cable (cost about 5 bucks at an LBS) and housing, if you haven't already.