Possible causes of shifting problems :

1. Rusty shifter cables.
2. Kinks in shifter housing.
3. Correct housing length - too short or too long will affect the shifting.
4. Do not use brake housing for the shifters, they are too flexible.
5. Shifter housing that are frayed.
6. Bent derailleur hanger.
7. Rapid fire shifters that have old grease that sticks to the ratcheting mechanism.
8. Rear derailleur spring tension that are stuck.
9. Rusty chains with stiff links that skips.
10. Rear derailleur rollers that are covered with dirt and old grease that prevent it from spinning smoothly.