Originally Posted by dsbrantjr

Did you route the rear derailleur shift cable properly with regards to the clamp screw and tab? The correct setup is not always intuitively obvious.

Did you make sure that the shifter was truly in the highest gear (most slack) position before you secured the cable?

You might want to start fresh by disconnecting the cable and following this procedure exactly, without skipping any steps: Park Tool Co. » ParkTool Blog » Rear Derailler Adjustments (derailleur)