Hi Shazbot - try the attached model and that should now work for those particular fillets that you want.

I tuned up this object by separating off the hood part, and then slicing off the bottom of it with a line drawn in the Front view so that the bottom area was actually a totally flat and simple plane object rather than a sweep or whatever that was close to being a plane but not quite all the way flat.

I then did an "untrim" of the now slightly too large opening on the main body (untrim is where you select a boundary of edges and then push Delete to remove them), and then re-booleaned the hood onto the main body to make the tuned up result.

Filleting in the geometry library that MoI uses tends to be somewhat sensitive to geometry that is almost planar but just a little slightly squished around instead of actually a regular plane.

Ideally you would not have to worry about that, but it usually helps to make things that are nearly flat to actually be exactly flat planes instead of only "sort of" flat.

For ends of things it can be good to slice them off with a line drawn in a side view so that you know the end is a plane.

- Michael