Filleting Issue

 From:  Rogurt
Hello all

I am having a hard time filleting the highlighted edge. Just wont work. The edge one step closer to the viewer does get filleted but not where the large fillet of the inner corner of the box is. The edge another step closer to the viewer fillets as supposed to (no screenshot).

Can anyone give me explanation on why it´s like that. And what can I do to get all the mentioned edges to fillet ok?