If the gap is at the top of the mast, thats a problem with either your hook, you knot on the ring being too big, or too many shackels in there. If it is between the foot of the mainsail and the boom, then there may not be enough to room there to down haul properly which will also be a problem.
Please explain a little more, or better yet, show us some pictures.

David Mosley
NACRA F18 Rigging Tips

The men were amazed, and said, "What kind of a man is this, that even the winds and the sea obey Him?" Matthew 8:27