With my build, the initial washers that I bought didn't fit over the unthreaded part of the nipple. This caused the nipple to move forward and backward. I had to buy different washers that fit over they unthreaded portion of the nipple (slightly larger inside diameter). Both washers were sold as the same size, but the inside diameters varied. This allowed me to tighten everything properly with no leaks.
I don't completely understand your issue, but I thought this might help.