A lot of times they leak where the flare sits inside the fitting. Try using grease or teflon tape. I have heard that a lot of the new fuel lines leak now a days.

NO tape, grease or teflon paste is recommended, plus a nice set of LINE WRENCHES, when you use a "regular" wrench on these fittings they distort the tube nut, basically egging it, so it dosen't seal/contact the flare 360 degrees, so it leaks....once egged, the fitting cannot be repaired and it will always leak no matter how tight you tighten it

As far as sixpack fuel lines, I use FINELINES or RIGHT STUFF STAINLESS lines...zero issues if installed properly, I've done quite a few sixpack set ups using their lines

Mike

Grease on the threads is an old mechanics trick that oftentimes does the job. The grease itself isn't to improve the sealing. The threads might have a burr or slight imperfection and when you tighten the lines, they may not tighten all the way due to the imperfection. The grease will lube the threads and allow the fitting to be tightened completely thus resulting in a tight seal with no leakage.
Doesn't always do the trick, but often it does, and it certainly is worth trying before going out and spending money on another set of lines!

I went with Fine Lines in non stainless for my 6 pack and all my fuel and brake lines, no leaks....took a few tries at my OE distribution block(pdb) to get it sealed using DOT5 fluid, but got it, as stated correct wrenches are a must, no galling, no slips and no scratches...no grease or teflon used either btw, just be careful and take your time and don't gorilla the fittings