hi mate, I grow NFT.
hard to say what your issue is. photos etc will help.
usually if you have everything ok the problem can be nutrients when in seedling stage. what are you feeding them from germ to going into NFT? EC values will help. it's important to start off really slow when introducing seedlings into NFT nutrients.
usually NFT will show signs of unhappiness quickly in your plants, for them to just die means things are pretty toxic in my opinion somewhere.
I agree that a well established root system increases your chances of success ten fold yet I have had success with a single tap root just touching the solution, but I generally wait for a good amount of roots before transfer.
pics of everything will help, also a list of equipment your running.
also im no NFT expert, but ive fucked up enough to know a thing or two.
also as mentioned earlier, get power sorted, ive had power issues last two grows, last one was mid flower, over 24hrs no power, no return for the plants, thought I fixed it this grow, another problem with power, have to get that sorted, fucking devastating man, months of hard work all stuffed from 24hrs no power. this time the plants made it at least but that doesn't happen often.