Still air does not circulate the air so the temperature is not distributed. You generally need to run it at a higher temperature - around 100-101 degrees. With a fan, the air is circulated so the eggs get a constant temperature across all of them.
With a fan you can keep the temperature at 99-100 degrees. I would put the fan in if you can.
Below is a picture of an egg on day 5 that I just took tonight. It is not completely focused, but if you look close you can see the veins on the right by the light. I placed 16 eggs on Saturday, lost one on Monday (cracked) and so far see veining in all but two eggs.