The reason fillet is having trouble on your shape is due to the sharp sort of "upsweep" shape right near the end:




That fairly sharp bend in shape near the end causes the fillet to need to mutate a lot in a short distance, things like that are generally not so good for getting good fillets.

One thing that will be particularly difficult is to get the fillet to extend properly in such a way that it intersects cleanly with the back face here:



With the fillet swooping close to being tangent to that back face it becomes difficult for it to intersect well with it.

It would probably work a lot better if the shape did not swoop so steeply in such a small area near the end there.

- Michael