Embryo Development?

Last night I candled my eggs and took out the duds. Hubby cracked them open so we could see if anything developed. 2 were just yolk, 2 were a bit of yolk with a purplish goo, and 1 had an embryo that only developed to about day 5 (I was on day 14 when I candled). What causes eggs to quit at a certain stage?

I'm trying to figure out this whole incubation thing, and I'd like to learn from mistakes to make this more successful. I'm using a little giant 9200. My temp stays b/n 100-102 and humidity has raning b/n 35%-50%. Thanks for you help!!

Sonja