Another possible problem:

You must make sure that the halyard starts out on the correct side of the hook as you raise the sail, otherwise it will never hook. Mine stays in place most of the time and works great. Then, on race day or when some hot beach Betty is watching, the line jumps unnoticed to the other side of the hook and I can't get it to hook without a lot of wasted time and labor.

Second tip:

I do NOT use a twist shackle. This aligns the ring fore and aft. After I raise the sail I rotate the mast and hook to starboard into the ring, then pull down to ensure it's hooked. To drop the sail I raise the sail one inch and the ring jumps off the hook...CLICK!. Rotate the mast to port to be certain but that's really not needed. This method never fails. Never...unless the halyard is on the wrong side of the hook to start.