I've got an 8 spd cassette on my main bike, and I've just changed the shifter. The gears aren't absolutely perfect at the best of times because it is a long tail bike so the cable is about 15cn/6" longer than usual and this does affect the responsiveness. This time, however, the shifting is fine in the lower and upper gears, but in the middle it either responds late or not at all untill I shift again, at which point it jumps a gear. This is the same going up or down, although down I can at least push the thumb shifter a bit to encourage it to change. What I don't understand is that apart from the middle it shifts like normal, so I don't see how it can be the tension. Any ideas?