Depending on which components it has, it may never shift prpoerly.

Try turning the barrel adjuster on the back of the RDER a 1/2turn at a time and see if it gets worse or better.
You may turn it so that ir seems better, but are missing a gear. In that case turn it in the opposite direction twice as much (give or take)

Look at this link-
http://www.parktool.com/blog/repair-help