Hi Leonard, there are some small-sized defects in the model that you may not be aware of.

For example zooming in to this area here:




When you get in really close (use the "Area" from the view tools at the bottom of the viewport) you can see it is actually not smooth at all there, it has like a tiny 90 degree shelf:



So that's going to limit the fillet to be some tiny fraction of that little shelf's width.

It looks like this may be some residual error from cutting the current rounded corners in using something like a boolean? It looks like the cutting object was not completely aligned to the sides... I would actually suggest avoiding placing those larger rounded corners by booleans and instead using Fillet to round them off as well with a larger radius.

I might be able to tune up your model, I'll give it a try.

- Michael