To make sure your jumbo prawns grill up to perfection, you want to have them all starting at the same temperature and ideally at room temperature. Don't leave them sitting out to spoil, but for best cooking results avoid putting them directly on the grill when they are cold (or worse frozen).
Jumbo prawns will grill up best when given this kind of head start - just be sure that you aren't leaving them warm for a long period before you start to grill the prawns.
Jumbo Prawns ready to Wok in a cool water rinse