Maybe if you increase the temperature you could possibly see better results. Is the pH of the influent near neutral? Also, have you attempted any alternative flocculants?
Depending on your current overflow rate you might want to consider installing lamella clarifiers also called tube settlers. They increase the projected surface area of the tank and reduce the settling path of the oil particles. The consequence is that the settling rate of the particles is increased up to 10 times. We have used tube settlers either for particle settling but also for oil coalescing and separation.
On Youtube are some videos that explain the functionality of tube settlers in more detail: https://youtu.be/wyz49PewhVk