King pigeons are one of the hardest that I've ever seen to hatch. Every time my friend hatches hers the parents have beat on it or abandoned the eggs midway. It might be awhile before they try to hatch squabs again but maybe hold out at least a day more to see what will happen
after 21 days I would throw them out yes young pairs some times take a few trys before getting it you can candle there eggs like chickens at 7 days and check them that way utilty kings are bred for production if they don't breed and raise a set of young once a month they are not doing there job maybe the twisted feathers friend had show king which are slow due to selection for show not production
Experienced pairs in my experience will only hover over the first egg and not set tight until the second egg is laid. This can make it seem as if it took 19 (first egg) or 20 days (second egg). Counting from the time that the first egg was laid.