fuel filler grommet

Did you clean the filler pipe? When I assemble that stuff, I Scotchbrite the last couple of inches, and then lube it with silicon grease. I also remove the grommet and clean both it and the tank opening. Even just a tiny piece of sand,in the wrong place, can be the source of seepage. Also,the pipe needs to fit fairly tight to the grommet. Thus it expands the grommet into the opening. With age, those grommets seem to shrink.
Good luck