Don't know if this is your issue or if the following will be helpful. What I often do if I have a relatively thin part that needs its shortest edges rounded is temporarily increase the thickness of the part so that the edges are much easier to select. Actually, most of my plate-type parts start from an already created 1x2x3 block, and if I know the corner edges are to be rounded, I do that before changing the dimensions to what I want.
