You can't simply rely on ratings to determine whether or not your offence will be successful. The ratings are determined by the combined skills of the players. If one of your players shot more poorly than the ratings suggest, then I think it may be down to one of three factors:
1. The player defending your shooter has good defence for the type of shooting he is defending against. This can be concealed in the defensive rating by its being combined with lower defence skills of his team mates, or by his being a reserve.
2. If you use an offensive scheme that does not focus on outside shooting, you won't take as many three-point shots compared to using Run and Gun.
3. The random factor at work again.