I would intervene if it has gone beyond 21 days. The inability to break out of the shell does not automatically mean the chick is weak or deformed. Sometimes they just get themselves in a bad position. I had an EE hen that lived to be 18 years old and if I had not decided to open the eggs as I was tossing them, she would never have "hatched". My father told me she would never amount of anything but she was a big, robust hen that laid all winter, long after the other hens stopped. He always had his egg for breakfast. It really is a personal decision.