The feed and speed, and/or step-down and step-over need to be adjusted. See the feeds and speeds tutorial for more information.
Turn the spindle speed up.
Adjust the feedrate, usually moving slower will be smoother, but this is not always the case.
Increase the step-down, taking a deeper cut into the material.
Increase or decrease the step-over, the width of the cut. Typically the ideal step-over is about 60% of tool diameter.