I agree. It sounds like your temps are a little low to begin with and probably have various cold spots. The float test, unless you see eggs wiggle independently isn't much help. Any egg with a big enough air cell to hold it up will float. I don't hold out much hope after day 23/24 either. I wouldn't pull the plug on a live chick, but after day 23 your chances of development issues increases. The further you get from "hatch day" the higher the possibilities of unhealthy chicks. I call it when I candle and see no movement.
You can wait as long as you want. I'd candle and the ones that show no lifeand I would tape into the air cell to double check and proceed with eggtopsies.