Just to make sure that the obvious things are ruled out, if using just a stainless ring and a shackle, make sure of the following:
1) You are using a capture ring of the appropriate size. If it looks too small, it might be...
2) You are attaching the ring with an appropriately sized *twist* shackle (if using an all-in-one ring-shackle, disregard points 1 and 2)
3) If your mast hook assembly is equipped with a mast locking loop, the halyard needs to go through the loop (over the head, through the loop, down the mast).
4) (and please don't be insulted if this does not apply; I have seen the following on about 5 occasions) make sure that the main halyard is rigged internally, down the mast slot, to the block at the base of the mast, (i.e. not external to the mast slot...)
5) make sure that the bolt rope is not jamming the ring onto the hook. Many times new sails need the bolt rope trimmed about 2 cm in order to release correctly.