Sometimes, the flare on the end of the line, where it fits against the fitting, can get flattened out over time, and will just not seal anymore. I've also seen them have small cracks in them right at the flares. Then you get a seepage from the line. Check to see that both sides of the connection are clean first, as has been suggested. IF the end of the fuel line itself where the flare is has flattened, the only real solution is to replace the line.
However, in emergency situations, I have used sewing thread to tighten the connection up, and stop the leak. Remove the line from the carb(or pump), and move the ferrel(nut) back from the flare. Wrap 6-7 wraps of sewing thread around the line, close to the flared end, clock-wise. Use a small amount of grease to hold the thread on. Then install the line as usual. This method has worked many times, but it is NOT meant to be a permanent fix, tho is does work 99% of the time. What it does is make the end of the flared portion a little thicker, as it was when new, so the nut(ferrel) will tighten down on the line nicely.
Teflon on fuel line fittings is usually a wasted effort.
Adams' Apple2010-10-21 09:33:58______________
Joel Adams
C3VR Lifetime Member #56
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"