That is not the float test to see if a chick is still alive inside. That test is just to see how much moisture an egg has lost. That test doesn't tell you if the egg is good to eat or not, just gives an idea of how old it is. It has nothing to do with an incubated egg, more if you find a hidden nest and wonder if the eggs are good to eat.
The type of float test to see if a chick is still alive in there should only be used as a last resort, just before you toss an egg you don't think will hatch. If an egg has a living chick in it and you float it in a still pan of water it will wiggle. You will clearly see it wiggling in the water. I hesitate to mention it because some people will see it as a way to check how the eggs are doing as they get close to hatch. All they are doing is creating an opportunity for things to go wrong but they do it anyway.
In your case I think it is justified. You want to know if you should toss that egg or or keep trying. If the egg wiggles put it back in the incubator and wait. If it doesn't wiggle it is dead. If you do that earlier in incubation you run the risk of washing off the bloom, which can allow bacteria to enter the porous egg, which will kill the chick. As late in the process as you are even if you rinse off bloom the chick will either hatch or not hatch before bacteria has a chance to be a problem.
what could have went wrong?
