27 days with chicken eggs and no chicks out, I'm sorry to say I wouldn't expect any to hatch at this point.
I surmise that you don't have an accurate thermometer. When temperature is 99.5 throughout incubation and all other parameters are ideal, they should pop out like popcorn on day 21.
I surmise that you don't have an accurate thermometer. When temperature is 99.5 throughout incubation and all other parameters are ideal, they should pop out like popcorn on day 21.