Locking the Hobie 17/18/20/21 Halyard:
It is best to simulate the halyard locking with the mast down so you clearly understand the system.
Then, depending on how old the boat is, be sure the hook does not have the old "flopper" stainless piece hanging on the hook. This old device caused difficulty in raising and hooking but would make it easier to release and lower.
Also, be sure that the knot tied to the ring is very low profile. A long bowline knot will hit the mast head before the ring gets to the hook.
If the ring has a small loop at the top... The line should be passed through the loop and a small knot tied. The knot (when ring and shackle are affixed to the sail) should be facing the mast. This tilts the ring closer to the mast.
Then (before attaching halyard shackle to the sail) spin the halyard 3 or 4 times clockwise (looking down on the shackle). This "pre-loads" the halyard line and causes the ring to swing back towards the hook. Keep the boat into the wind and hoist. Should lock easily.
To release... fully release the downhaul and outhaul. Partially feed the sail up the luff track. Hoist with the halyard to the top till it stops, hold... rotate the aft of the mast base to starboard, hold the mast rotated, ease the halyard a few feet before releasing the mast. Lower the sail.