If the first one hatched on day 18 and the others are showing no action I would candle them to see what is going on in them. The first chick that hatched should be ready now for food and water as it's been three days since hatch. Have some available near by so mother hen can show the chick what to do. If the eggs are showing no internal or external pips or you can't hear any peeping coming from the eggs I would remove them and let mother hen take care of her single chick.
Hope when you do candle they are not far from hatching, is it possible that other hens could have been adding eggs to the nest after she began sitting? This could explain why they are so delayed compared to the first hatcher. But then again on the other hand 21 days is just a guide and not set in stone. Also smaller bantam eggs can hatch earlier then large fowl eggs. Are all your eggs of the same breed?