Your sail must not be locked in at the top. If the halyard is properly locked... it can not slip down.
If you rely on halyard tension alone... sailing loads, mast bend etc could cause it to slip down a bit.
Locking the Hobie 14/16/Getaway halyard:
Once the sail is fully hoisted (be sure that the sail is fully inserted into the feeder). Pull the halyard forward of the mast by 3-4 feet. Hold the halyard on the centerline of the mast. Pull hard and hold the tension while bringing the halyard into the mast. Release the halyard tension and see that the sail remains fully hoisted. This seats a small bead, in the halyard, under a two finger prong "hook" and the top of the mast. If the sail slips down when downhaul tension is added, repeat the final hoist technique again. Be sure the bead is clear to pass the hook before pulling tension on the halyard.
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... pull the halyard line forward 3-4 feet away from the mast, ease the halyard to lower the sail.
Halyard locking is explained at 1:20 and on.