It is the cable stretch which causes the indexing problem.
Sometimes you can take up the slack with the various cable stops, but often you have to pull the whole cable a bit tighter.
Your LBS should do that free, its a 2 min job if you know how, and ther best way to learn is to watch a decent mechanic do it.

Dont touch the two limit screws on the derailleur mechanism, these are NOT affected by cable tension.