You may want to get some 48 inch fence, bend it in half lengthwise, into an L shape and fasten it to the bottom of the other fence. Cover the part on the ground with soil or rocks. The rabbits may dig to get under the fence and hit the buried fence. They won't figure it out in their lifetime.
If the fence doesn't work (the lifetimes of rabbits is getting longer, too.) you might try some of the animal urine scents available at nursery centers. I've used coyote urine, applying a drop or two to strips of cloth attached to the fence every 10 feet.
Norm

Good idea Norm

Thanks,

Al