Your SG is having "Average" game shape, what is not so good. If you increase it, making him play between 48 and 70 minutes aprox., he will improve his fg%.
Anyway, reason can also be related to the other teams in your group. Maybe there are less bots, the other guys have also improved their rosters, maybe they already know this guy is your best shooter so they use their best defender against him...
As you can see, there are many factors to have in consideration ;)