Hi Toha, there are a couple of places where you've got 2 surfaces adjacent to each other which are close to being smooth but not quite all the way smooth:





That can make things difficult for filleting because surfaces that meet at a very shallow angle like 3 or 4 degrees will try to get a fillet made between them but it will be a very very skinny fillet and it will have difficulty intersecting a very skinny piece with the rest of the model.

I'll see if I can tune it up for you. The first part will be to draw a line and do a boolean difference on the bottom part to shave off a piece here so the bottom can be all one single plane:



- Michael