Hi, i just changed out the gear cables on an old specialized "hard rock" mtb. The problem is that when i shift nothing happens, the twist shifters click and move fine and there is tension in the cables, but the gears don't actually shift.
Try this; put the shifter in the position that lets all of the cable out of the shifter. If there is any slack in the cable loosen the cable fixing bolt and pull all of the slack out of the cable. Reattach. You may also need to add a little tension to the cable using the adjuster.
Good shifter set up instructions can be found at www.parktool.com.
2008 Novara Randonee - love it. Would have more bikes if I had time to ride them all. Previous bikes: 1968 Motobecane Mirage, 1972 Moto Grand Jubilee (my fav), Jackson Rake 16, 1983 C'dale ST500.
To start at the simplest point, grab hold of the cable where it emerges from the housing and make sure it moves when you operate the shifters. If not you installed the cable incorrectly in the shifter! If that's OK then the previous suggestion should work.