When you say "highest" do you mean "highest gear ratio" (ie smallest sprocket) or "highest" in the sense of "furthest up the cassette" (ie biggest sprocket)? If it's the smallest sprocket, the cable tension's probably too high - turn the barrel adjuster (on the mech if it's conventional, on the shifter if it's a Shadow) clockwise 1/4 turn at a time until it's happy. If it's the biggest sprocket, the reverse is true, although big ring/big sprocket is a nasty gear to be in anyway.
