Hi Anthony, welcome to MoI!

Fillet is one of the most sensitive (and prone to failure) operations that MoI tries to do.

It involves several complex operations in sequence, like calculating surface offsets, and extensions, and then intersections between those pieces. If there are any difficulties in any one of those stages it can prevent the fillet from being formed.

So for filleting it tends to help to have a very clean and regular shaped input into it. Any irregularities in your base objects tend to get magnified and cause problems when trying to fillet them.

It looks like in your case here, the main problem area is this spot:





If you look closely, you can see that the curves are not smooth there, they actually come to a corner point but it is a shallow one, the curve tangents have a difference of 15 degrees between them there.

That kind of situation where things look pretty close to being smooth but actually are not smooth and have a shallow crease between them will tend to cause problems with filleting, it is difficult for the intersection part to work properly in situations like that.

You want those kinds of junctures to either be actually smooth to one another or to have more of a pronounced corner than 15 degrees.

- Michael