If after staring at it, cleaning it, adjusting it, lubing it and using a hammer on it it still doesn't work well, replace the cable and housing. I had a thread about similar issues about a month ago and everyone kept saying to replace the cable. After trying everything else I replaced the cable (luckily I found a new cable/housing set in my basement!) and all is good. My bike was a lot older though and left out in the weather for a few years, a 1990 Trek. It doesn't take much rust or dirt to completely gum up the housing.