what are you feeding them? Are they normals or another variety? I don't want to jinx myself, but I could count on one hand the number of deaths I've had in probably 2,000 hatchlings. My guess would be either an issue with food, mold, or illness... and all three of those things go hand in hand. They have no real immunity so mold and bacteria can set in and make them sick very quickly.
I've had good success with keeping them in an open container, with enough space that they don't need to crawl all over each other... only a little food provided each day, just enough to either be eaten, or completely dry up by the next feeding. That also lets the frass fall to the bottom and dry out... keeping the risk of mold and bacteria to a minimum. As others have said, once you remove them from the bin, don't put them back in it. I'm not as fanatical about disinfecting every single thing that comes into contact with the silks, but I do wash my hands and keep their food as clean as I can.