Can I ask if you have reflectivity on the material...?
If not have you checked the radius setting for final gather?
The best way to do this to get a correct scale for the setting is to put a poly plane in the scene and scale it up to just over the objects size, then check the scale setting in the channel box.
Then divide that number by ten for your Max Setting and then divide the Max setting by a tenth to get your min setting. this should do it, the only other alternative is to keep upping the accuracy like you have done until the FG rays disappear.