Do you use exclude by Object or Spline ?
The exclude by object is based in a bitmap mask which Forest builds internally. This mode is less precise than a spline, specially using large objects. I mean, it may work fine for a house, but worse for a large road. Although you can increase the resolution of the bitmap (see the area parameters), in this case i suggest to use an area spline. It's much faster and precise.
Besides that, try setting Area->Boundary Checking to Size. That may help.