Has it ever worked with the cut you're using?
Try deleting (or renaming) your prefs, then restart the app to see if that helps.
Pref locations:
On windows: C:\Documents and Settings\[your account]\Application Data\Autodesk\SketchBook Pro
On mac: /Users/[your user]/Library/Application Support/Autodesk/SketchBook Pro/4.0
c.