Cliff has a good point. If you live in an area with PH below 7, you need to raise it a bit or they won't hatch well, that's what the baking soda is for (I've heard around 2 level teaspoons per litre, but maybe Cliff can clarify on that one) . When I measured my salt I used 1.5 to 2 level tablespoons (those measuring spoons that you use for baking) per litre of water and per 1/2 a level teaspoon of eggs. The only reason I heated my water is because in the south east, my flat gets to about 5 C in the winter (the call to arms for socks!) so they weren't hatching at all. I stuck my home made hatcheries in a bucket of water so the tops of the hatcheries were above the water (the necks of the hatcheries bottles were held in place by suction cups with clips on that you use for heaters), then I heated the bucket with a small aquarium heater.
The initial batch I bought never hatched no matter how many batches I did, but the new ones I got all hatched under the exact same conditions. Sometimes it can depend on how the seller stores them as to whether they hatch at all. You should keep dry unhatched eggs in an airtight container in the fridge when in use.
If they're floating when you put them in, you should stir the water a bit and use a clean finger to gently wipe any down from the sides. They won't hatch if they're not underneath the water! Though the hatched shells will float too, so after about 8 hours you may see some of those start to appear at the top that don't need to be stirred around.