Tighten the leech line
It's difficult --you have to be an acrobat and climb out on the arch....
or furl it in, tighten, unfurl.
If it doesn't work-repeat until it does.
I've also found that tightening down hard on the mainsheet helps a lot too.
It's tough with the boom hanging way aft (and dangerous). Be careful if you opt for the first way.