It sounds like the Nacra has the same hook as the Hobie 21 and 18, if it does then I've seen people get hung up by:
1: Not releasing the downhaul enough. Take it off all the way. You can also put your hands flat on either side of the sail at the luff while standing in front of the mast and pushing the sail up the track a little so you aren't trying to pull the entire luff up.
2: Trying to raise the sail to clear the hook with the sail at an angle to the mast. If the sail isn't coming straight out of the trailing edge of the mast it can bind and be very difficult to pull the sail up.

John