I was helping with a Nacra having this problem at my last regatta and couldn't make any of the ideas expressed in this thread work. We tipped the boat on its side to see what was going on and his halyard had come off the shiv at the top of the mast. We put the halyard back on the shiv and it worked great. He purchased a new shiv to prevent the halyard from coming off again.

I have been using this sytem on my boats for the last 20 years and the suggestions in this thread always worked until this experience. It is the first time I experienced the halyard coming off the shiv.

Just one more thing to take a look at.