It is good to give them three days past the due day from what I have been reading here on BYC as many folks have hatches during that time frame. (Of course this includes candling them to see what is going on and you did the float test).
The float test doesn't really show you if there is a viable chick inside. Candle the egg to see if there is movement. Then wiggle the egg around lightly to see if you hear peeping. But at day 23 you're getting to the end, another day and you have the risk of an exploding egg.
Well when mine are like that I usually will "pip" them myself in the large end where the air pocket is to check whether or not its alive. I wouldn't trust the float test, only candle or pip. Also I wouldn't throw it away if its completely dark (except for the air pocket) when candled in case it's still a lie, and instead (like i said) pip it first above the air pocket to avoid hitting the membrane. If you do that though you have to keep the membrane moist (if its alive) so it doesn't shrink wrap. But past a certain point they'll suffocate. if they're not cheeping I have a bad feeling they're not alive. But candle them… =(
I usually pip them at the air cell. If there is no movement or chirping, I know they're dead. I had about six that were fully developed but dead this past hatch.
I also had two that were shrink-wrapped for some reason. They pipped but couldn't go any farther. I ended up helping them (which I never do). But this time I took them, shell and all, and ran them under warm water and gently peeled off the shell and membrane. They're both doing great! Had I left them, they would have died.