Your problem is indeed caused by the low GS of your players. Try to get 60-65 minutes played on your players every week and they should have 8-9 GS and perform better as well as produce better ratings.
Low HA and PA on your bigs an also lead to more TO's. But that won't show in the ratings.
Thanks. I started posting but then I realized I didn't really know enough to be useful. What about driving?
Driving: Allows the player to create his own shot without a good pass from a teammate. Better driving creates shots more often and creates better shots on average. I don't know if driving will raise the Offensive Flow rating (it sounds like it could), but I do know it helps with open looks on offense. I'd guess that most players with high handling also have high driving anyway.