Only thing I can figure is either it is cross threaded (remove and inspect line) or you don't have the line tight enough. When installing the block, get both lines started first so they are threading smoothly and then bolt block down. Finally tighten lines.
If it were leaking at the caliper, I'd also ask if the new copper washers were used on both sides of the banjo fitting and they didn't get dirty or scratched when installing. I saw a leak once and it turned out to be a small scratch on one of those copper washers.