The derailleur moves the chain to set points - over the chainring. There are stops that limit the inner and outer movement and those are set with screw adjustment. Cables stretch from use, usually when new. That tends to have the largest impact on the middle chainring of a triple set up. You can check it out and make adjustmenst per here:

http://www.parktool.com/repair/readhowto.asp?id=75