Is it a new hose? I had that leak for years. Two things happen...the nut gets cinched down too tight and it scores the brass cone insert inside the box and/or the flare end of the hose gets deformed and doesn't seat right. First try a new hose and becareful it doen't move when you tighten it down. If that doesn't do it, I believe you can get a new brass insert cone and R&R that. One or the other or both should fix it.