I know, sounds like a stupid question.
Is all the slack out of the cable? If you're not sure, loosen the set screw, and pull on the cable with a needle nose. Actually pull the throttle up, and then let it go back just to where it's closed, and tighten the set screw.
If this isn't the problem, you probably had it right the first time. The problem is most likely in the twist grip itself.