The other problem area is where these edges meet up with one another:



Those 2 edges there are not quite smooth to each other - they're nearly smooth but actually have a slight 1 degree or so angle crease in their end tangents where they meet.

That kind of "nearly smooth but just not quite" type of shape is also difficult for filleting, when pieces are not smooth to each other often times the fillets constructed there do not naturally meet up without some kind of corner juncture being made, but when they are so slightly angled the corner juncture becomes a little slivery thing that's difficult to trim and cut properly.

Also as the fillet radius becomes larger the fillet sticks out more to either side of the juncture area and the small tangent deviation becomes more problematic...

So if you want to get a fillet of larger than 1.5 on that, you may need to redraw those pieces to be more accurately smooth in that spot. Either that or have it come to a somewhat more distinct sharper corner and let it get rounded off by filleting as well. Filleting basically likes this to be either really smooth to one another or fairly more distinctly sharp to each other, when it's "almost smooth but not quite" like you've got here (like maybe up to 4 angular degrees or so tangent deviation say) that tends to be problematic for filleting.

- Michael