I'm not understanding how this fitting works.... I'm used to seeing a compression fitting with a big nut over the (left) copper side of the fitting... this fitting looks like the brass side is one solid piece, with a dielectric plastic sleeve inside.... it has developed a small leak on the right (stainless?) side where the stainless nut holds the flex line. Is this one of those new push on fittings perhaps?
I tried tightening the brass/stainless junction holding a plier on the brass nut and a pliers on the stainless nut and applied opposing force.. the nut went maybe an 1/16 or an 1/8 turn and that was about it. Leak is still there. I noticed that the left side (brass/dielectric) rotated about the copper pipe a bit when I applied the pliers to the brass nut.
Pictures below:
-- Paul