Foliage doesn't fill all my 3D moebius ring

Hello guys and girls,

I request your help for a little project : my goal is to add grass all around this moebius ring. But even if I change parameters I can’t fill it fully.
Look at this picture bellow, It seems that it can’t fill on the twisted parts even if I change the “max ground Slope Angle” parameter.

Does someone know how I can do?

Thanks for your help !