I can usually see internal pipping when I candle at this stage (when they are due to hatch). Internal pipping is when they've broken thru the membrane and the tip of their beak is inside the air cell, up against the inside of the shell (mostly you just see the shadow and shape), but if you rotate the egg slowly while candling you can usually see the keet wiggling a little inside if it is alive. Not always, but you may be able to see something that gives you some hope. I'd bump up the humidity a little more, closer to 80%... and give them a quick fine mist of water over the top before you lock back down after candling. If your temps dropped a few degrees it may have slowed the hatch... I wouldn't give up hope until around day 30...