Different developing rates?

[ahoyaki]

7 days ago I bought 12 quail eggs, 2 weren't fertile, and the rest were developing, however, I noticed that some were at the right stage of the development process while some others were a bit behind by maybe a day, they didn't stop developing as I could see the heart beat, but one seems to still be on day 5 of the development process, I can't see the heart beat because of the pattern of the egg shells and I'm a bit worried that it quit. I'm hoping that the egg is just a bit of a slow starter instead of a quitter. Oh and also I'm using a little giant still air incubator with a spare computer fan that I put in the middle thats blowing at the heating element.

 

[sourland]

I'm wondering if temperatures may differ throughout your incubator causing the eggs to develop at different speeds.

 

[ahoyaki]

I'm wondering if temperatures may differ throughout your incubator causing the eggs to develop at different speeds.

I've got 3 thermometers in the incubator and since I added the fan all the thermometers have been reading the same humidity/heat